diff --git a/eng/native/configurecompiler.cmake b/eng/native/configurecompiler.cmake
index 1d446066476f3e..656a853f44ab75 100644
--- a/eng/native/configurecompiler.cmake
+++ b/eng/native/configurecompiler.cmake
@@ -325,9 +325,16 @@ if(CLR_CMAKE_HOST_LINUX)
   add_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--noexecstack>)
   add_linker_flag(-Wl,--build-id=sha1)
   add_linker_flag(-Wl,-z,relro,-z,now)
-elseif(CLR_CMAKE_HOST_FREEBSD OR CLR_CMAKE_HOST_OPENBSD)
+elseif(CLR_CMAKE_HOST_FREEBSD)
   add_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--noexecstack>)
   add_linker_flag("-Wl,--build-id=sha1")
+elseif(CLR_CMAKE_HOST_OPENBSD)
+  add_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--noexecstack>)
+  add_linker_flag("-Wl,--build-id=sha1")
+  # OpenBSD's ld.so can't resolve native TLS relocs in a .so; rely on clang's default
+  # emulated TLS (don't pass -fno-emulated-tls).
+  # The PAL's hand-written asm lacks endbr64 landing pads, so disable branch-target CFI.
+  add_linker_flag("-Wl,-z,nobtcfi")
 elseif(CLR_CMAKE_HOST_SUNOS)
   add_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--noexecstack>)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fstack-protector")
@@ -847,6 +854,20 @@ else(CLR_CMAKE_TARGET_UNIX)
   add_compile_definitions($<$<NOT:$<BOOL:$<TARGET_PROPERTY:IGNORE_DEFAULT_TARGET_OS>>>:TARGET_WINDOWS>)
 endif(CLR_CMAKE_TARGET_UNIX)
 
+# OpenBSD only exposes its libunwind symbols through the LLVM C++ runtime
+# (libc++/libc++abi), so default to those there. An explicit selection via
+# CLR_CMAKE_CXX_STANDARD_LIBRARY/CLR_CMAKE_CXX_ABI_LIBRARY (applied by the cross
+# toolchain file) still takes precedence.
+if(CLR_CMAKE_TARGET_OPENBSD)
+  if(NOT CLR_CMAKE_CXX_STANDARD_LIBRARY)
+    add_compile_options($<$<COMPILE_LANG_AND_ID:CXX,Clang>:--stdlib=libc++>)
+    add_link_options($<$<LINK_LANG_AND_ID:CXX,Clang>:--stdlib=libc++>)
+  endif()
+  if(NOT CLR_CMAKE_CXX_ABI_LIBRARY)
+    add_link_options("LINKER:-lc++abi")
+  endif()
+endif()
+
 if(CLR_CMAKE_HOST_UNIX_ARM)
    if (NOT DEFINED CLR_ARM_FPU_TYPE)
      set(CLR_ARM_FPU_TYPE vfpv3)
diff --git a/eng/native/functions.cmake b/eng/native/functions.cmake
index f7066de09a764c..5064f51b101220 100644
--- a/eng/native/functions.cmake
+++ b/eng/native/functions.cmake
@@ -192,7 +192,13 @@ function(find_unwind_libs UnwindLibs)
     find_library(UNWIND NAMES unwind)
 
     if(UNWIND STREQUAL UNWIND-NOTFOUND)
-      message(FATAL_ERROR "Cannot find libunwind. Try installing libunwind8-dev or libunwind-devel.")
+      if(CLR_CMAKE_TARGET_OPENBSD)
+        # On OpenBSD the libunwind symbols are provided by the C++ ABI library
+        # (libc++abi), so a standalone libunwind is not expected.
+        set(UNWIND "")
+      else()
+        message(FATAL_ERROR "Cannot find libunwind. Try installing libunwind8-dev or libunwind-devel.")
+      endif()
     endif()
 
     set(${UnwindLibs} ${UNWIND_LIBS} ${UNWIND} PARENT_SCOPE)
diff --git a/src/coreclr/gc/unix/gcenv.unix.cpp b/src/coreclr/gc/unix/gcenv.unix.cpp
index 575d9cfd9f8ccb..427aecd42e3de1 100644
--- a/src/coreclr/gc/unix/gcenv.unix.cpp
+++ b/src/coreclr/gc/unix/gcenv.unix.cpp
@@ -1013,11 +1013,13 @@ static size_t GetCurrentVirtualMemorySize()
 //  non zero if it has succeeded, GetVirtualMemoryMaxAddress() if not available
 size_t GCToOSInterface::GetVirtualMemoryLimit()
 {
+#ifdef RLIMIT_AS
     rlimit addressSpaceLimit;
     if ((getrlimit(RLIMIT_AS, &addressSpaceLimit) == 0) && (addressSpaceLimit.rlim_cur != RLIM_INFINITY))
     {
         return addressSpaceLimit.rlim_cur;
     }
+#endif // RLIMIT_AS
 
     // No virtual memory limit
     return GetVirtualMemoryMaxAddress();
diff --git a/src/coreclr/hosts/corerun/corerun.hpp b/src/coreclr/hosts/corerun/corerun.hpp
index ec5a978814ec3c..03187959f88b49 100644
--- a/src/coreclr/hosts/corerun/corerun.hpp
+++ b/src/coreclr/hosts/corerun/corerun.hpp
@@ -384,6 +384,16 @@ class platform_specific_actions final
 #include <link.h>
 #include <elf.h>
 #include <cstring>
+
+// glibc defines the ElfW() macro to select the native-width Elf type, but some
+// libcs (e.g. OpenBSD) don't provide it. Fall back to the appropriate fixed-width type.
+#ifndef ElfW
+#if defined(TARGET_64BIT)
+#define ElfW(type) Elf64_##type
+#else
+#define ElfW(type) Elf32_##type
+#endif
+#endif // ElfW
 #endif
 
 // CMake generated
diff --git a/src/coreclr/inc/check.h b/src/coreclr/inc/check.h
index b020f5f3b6f067..826bb46592ee03 100644
--- a/src/coreclr/inc/check.h
+++ b/src/coreclr/inc/check.h
@@ -1,9 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
+
 // ---------------------------------------------------------------------------
 // Check.h
-//
-
 //
 // Assertion checking infrastructure
 // ---------------------------------------------------------------------------
@@ -663,7 +662,7 @@ CHECK CheckAligned(UINT value, UINT alignment);
 CHECK CheckAligned(ULONG value, UINT alignment);
 #endif
 CHECK CheckAligned(UINT64 value, UINT alignment);
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 CHECK CheckAligned(SIZE_T value, UINT alignment);
 #endif
 CHECK CheckAligned(const void *address, UINT alignment);
@@ -673,7 +672,7 @@ CHECK CheckOverflow(UINT value1, UINT value2);
 CHECK CheckOverflow(ULONG value1, ULONG value2);
 #endif
 CHECK CheckOverflow(UINT64 value1, UINT64 value2);
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 CHECK CheckOverflow(SIZE_T value1, SIZE_T value2);
 #endif
 #ifndef __wasm__
@@ -689,7 +688,7 @@ CHECK CheckUnderflow(UINT value1, UINT value2);
 CHECK CheckUnderflow(ULONG value1, ULONG value2);
 #endif
 CHECK CheckUnderflow(UINT64 value1, UINT64 value2);
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 CHECK CheckUnderflow(SIZE_T value1, SIZE_T value2);
 #endif
 CHECK CheckUnderflow(const void *address, UINT offset);
@@ -697,7 +696,7 @@ CHECK CheckUnderflow(const void *address, UINT offset);
 CHECK CheckUnderflow(const void *address, ULONG offset);
 #endif
 CHECK CheckUnderflow(const void *address, UINT64 offset);
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 CHECK CheckUnderflow(const void *address, SIZE_T offset);
 #endif
 CHECK CheckUnderflow(const void *address, void *address2);
diff --git a/src/coreclr/inc/check.inl b/src/coreclr/inc/check.inl
index b0f65c5d218bbd..ad2952d0cb8930 100644
--- a/src/coreclr/inc/check.inl
+++ b/src/coreclr/inc/check.inl
@@ -156,7 +156,7 @@ inline CHECK CheckAligned(UINT64 value, UINT alignment)
     CHECK_OK;
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline CHECK CheckAligned(SIZE_T value, UINT alignment)
 {
     STATIC_CONTRACT_WRAPPER;
@@ -192,7 +192,7 @@ inline CHECK CheckOverflow(UINT64 value1, UINT64 value2)
     CHECK_OK;
 }
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 inline CHECK CheckOverflow(SIZE_T value1, SIZE_T value2)
 {
     CHECK(value1 + value2 >= value1);
@@ -237,7 +237,7 @@ inline CHECK CheckOverflow(const void *address, UINT64 offset)
     CHECK_OK;
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline CHECK CheckOverflow(const void *address, SIZE_T offset)
 {
     CHECK((UINT64) address + offset >= (UINT64) address);
@@ -271,7 +271,7 @@ inline CHECK CheckUnderflow(UINT64 value1, UINT64 value2)
     CHECK_OK;
 }
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 inline CHECK CheckUnderflow(SIZE_T value1, SIZE_T value2)
 {
     CHECK(value1 - value2 <= value1);
@@ -316,7 +316,7 @@ inline CHECK CheckUnderflow(const void *address, UINT64 offset)
     CHECK_OK;
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline CHECK CheckUnderflow(const void *address, SIZE_T offset)
 {
     // SIZE_T is 32bit on wasm32
@@ -371,4 +371,3 @@ inline CHECK CheckBounds(const void *rangeBase, UINT32 rangeSize, UINT32 offset,
 }
 
 #endif  // CHECK_INL_
-
diff --git a/src/coreclr/inc/clrtypes.h b/src/coreclr/inc/clrtypes.h
index 80901d8fda5925..546531eb58184c 100644
--- a/src/coreclr/inc/clrtypes.h
+++ b/src/coreclr/inc/clrtypes.h
@@ -1,5 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
+
 // ================================================================================
 // Standard primitive types for CLR code
 //
@@ -275,7 +276,7 @@ inline UINT64 AlignUp(UINT64 value, UINT alignment)
     return (value+alignment-1)&~(UINT64)(alignment-1);
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline SIZE_T AlignUp(SIZE_T value, UINT alignment)
 {
     STATIC_CONTRACT_LEAF;
@@ -316,7 +317,7 @@ inline uintptr_t AlignDown(uintptr_t value, UINT alignment)
 }
 #endif
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 inline SIZE_T AlignDown(SIZE_T value, UINT alignment)
 {
     STATIC_CONTRACT_LEAF;
@@ -345,7 +346,7 @@ inline UINT AlignmentPad(UINT64 value, UINT alignment)
     return (UINT) (AlignUp(value, alignment) - value);
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline UINT AlignmentPad(SIZE_T value, UINT alignment)
 {
     STATIC_CONTRACT_WRAPPER;
@@ -378,7 +379,7 @@ inline UINT AlignmentTrim(UINT64 value, UINT alignment)
     return ((UINT)value)&(alignment-1);
 }
 
-#if defined(__APPLE__) || defined(__wasm__)
+#if defined(__APPLE__) || defined(__wasm__) || defined(__OpenBSD__)
 inline UINT AlignmentTrim(SIZE_T value, UINT alignment)
 {
     STATIC_CONTRACT_LEAF;
diff --git a/src/coreclr/inc/pedecoder.h b/src/coreclr/inc/pedecoder.h
index a373f90d55d3b1..1a16bc39f1f4c8 100644
--- a/src/coreclr/inc/pedecoder.h
+++ b/src/coreclr/inc/pedecoder.h
@@ -102,6 +102,8 @@ inline CHECK CheckOverflow(RVA value1, COUNT_T value2)
 #define IMAGE_FILE_MACHINE_NATIVE_OS_OVERRIDE 0x4644
 #elif defined(__FreeBSD__)
 #define IMAGE_FILE_MACHINE_NATIVE_OS_OVERRIDE 0xADC4
+#elif defined(__OpenBSD__)
+#define IMAGE_FILE_MACHINE_NATIVE_OS_OVERRIDE 0xADC5
 #elif defined(__linux__)
 #define IMAGE_FILE_MACHINE_NATIVE_OS_OVERRIDE 0x7B79
 #elif defined(__NetBSD__)
diff --git a/src/coreclr/jit/compiler.hpp b/src/coreclr/jit/compiler.hpp
index 98f1ff991d710b..7e0b3dd93a674d 100644
--- a/src/coreclr/jit/compiler.hpp
+++ b/src/coreclr/jit/compiler.hpp
@@ -141,12 +141,12 @@ inline unsigned genLog2(uint64_t value)
     return BitOperations::BitScanForward(value);
 }
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 inline unsigned genLog2(size_t value)
 {
     return genLog2((uint64_t)value);
 }
-#endif // __APPLE__
+#endif // __APPLE__ || __OpenBSD__
 
 // Given an unsigned 64-bit value, returns the lower 32-bits in unsigned format
 //
diff --git a/src/coreclr/jit/jit.h b/src/coreclr/jit/jit.h
index 5c7f32e258598d..7f2eb2ff8e0c24 100644
--- a/src/coreclr/jit/jit.h
+++ b/src/coreclr/jit/jit.h
@@ -731,12 +731,12 @@ inline size_t unsigned_abs(ssize_t x)
     return ((size_t)std::abs((int64_t)x));
 }
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 inline size_t unsigned_abs(int64_t x)
 {
     return ((size_t)std::abs(x));
 }
-#endif // __APPLE__
+#endif // __APPLE__ || __OpenBSD__
 #endif // TARGET_64BIT
 
 /*****************************************************************************/
diff --git a/src/coreclr/minipal/Unix/doublemapping.cpp b/src/coreclr/minipal/Unix/doublemapping.cpp
index 26faf9c50cacdd..6c7619c55fa26e 100644
--- a/src/coreclr/minipal/Unix/doublemapping.cpp
+++ b/src/coreclr/minipal/Unix/doublemapping.cpp
@@ -1,6 +1,5 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
-//
 
 #include <stddef.h>
 #include <sys/mman.h>
@@ -102,6 +101,8 @@ bool VMToOSInterface::CreateDoubleMemoryMapper(void** pHandle, size_t *pMaxExecu
     // Clip the maximum double mapped memory size to 1/4 of the virtual address space limit.
     // When such a limit is set, GC reserves 1/2 of it, so we need to leave something
     // for the rest of the process.
+#ifdef RLIMIT_AS
+    // OpenBSD has no address-space rlimit (RLIMIT_AS), so this clipping is skipped there.
     struct rlimit virtualAddressSpaceLimit;
     if ((getrlimit(RLIMIT_AS, &virtualAddressSpaceLimit) == 0) && (virtualAddressSpaceLimit.rlim_cur != RLIM_INFINITY))
     {
@@ -111,6 +112,7 @@ bool VMToOSInterface::CreateDoubleMemoryMapper(void** pHandle, size_t *pMaxExecu
             maxDoubleMappedMemorySize = virtualAddressSpaceLimit.rlim_cur;
         }
     }
+#endif // RLIMIT_AS
 
     // Clip the maximum double mapped memory size to the file size limit
     struct rlimit fileSizeLimit;
diff --git a/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets b/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets
index 8c429c993788bb..e927956fd04928 100644
--- a/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets
+++ b/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets
@@ -21,6 +21,7 @@ The .NET Foundation licenses this file to you under the MIT license.
     <CppLibCreator>ar</CppLibCreator>
     <_SymbolPrefix Condition="'$(_IsApplePlatform)' == 'true'">_</_SymbolPrefix>
     <LinkerFlavor Condition="'$(LinkerFlavor)' == '' and '$(_targetOS)' == 'freebsd'">lld</LinkerFlavor>
+    <LinkerFlavor Condition="'$(LinkerFlavor)' == '' and '$(_targetOS)' == 'openbsd'">lld</LinkerFlavor>
     <LinkerFlavor Condition="'$(LinkerFlavor)' == '' and '$(_linuxLibcFlavor)' == 'bionic'">lld</LinkerFlavor>
     <LinkerFlavor Condition="'$(LinkerFlavor)' == '' and '$(_targetOS)' == 'android'">lld</LinkerFlavor>
     <LinkerFlavor Condition="'$(LinkerFlavor)' == '' and '$(_targetOS)' == 'linux'">bfd</LinkerFlavor>
@@ -65,6 +66,7 @@ The .NET Foundation licenses this file to you under the MIT license.
       <TargetTriple Condition="'$(CrossCompileArch)' != '' and '$(_IsAlpineExitCode)' != '0'">$(CrossCompileArch)-linux-$(CrossCompileAbi)</TargetTriple>
       <TargetTriple Condition="'$(CrossCompileArch)' != '' and '$(_IsAlpineExitCode)' == '0'">$(CrossCompileArch)-alpine-linux-$(CrossCompileAbi)</TargetTriple>
       <TargetTriple Condition="'$(CrossCompileArch)' != '' and ($(CrossCompileRid.StartsWith('freebsd')))">$(CrossCompileArch)-unknown-freebsd12</TargetTriple>
+      <TargetTriple Condition="'$(CrossCompileArch)' != '' and ($(CrossCompileRid.StartsWith('openbsd')))">$(CrossCompileArch)-unknown-openbsd</TargetTriple>
 
       <SharedLibraryInstallName Condition="'$(SharedLibraryInstallName)' == '' and '$(_IsApplePlatform)' == 'true' and '$(NativeLib)' == 'Shared'">@rpath/$(NativeBinaryPrefix)$(TargetName)$(NativeBinaryExt)</SharedLibraryInstallName>
 
@@ -222,14 +224,14 @@ The .NET Foundation licenses this file to you under the MIT license.
 
     <ItemGroup>
       <NativeSystemLibrary Include="stdc++" Condition="'$(LinkStandardCPlusPlusLibrary)' == 'true'" />
-      <NativeSystemLibrary Include="dl" />
+      <NativeSystemLibrary Include="dl" Condition="'$(_targetOS)' != 'openbsd'" />
       <NativeSystemLibrary Include="objc" Condition="'$(_IsApplePlatform)' == 'true'" />
       <NativeSystemLibrary Include="swiftCore" Condition="'$(_IsApplePlatform)' == 'true'" />
       <NativeSystemLibrary Include="swiftFoundation" Condition="'$(_IsApplePlatform)' == 'true'" />
       <NativeSystemLibrary Include="z" Condition="'$(UseSystemZlib)' == 'true'" />
       <NativeSystemLibrary Include="brotlienc;brotlidec;brotlicommon" Condition="'$(UseSystemBrotli)' == 'true'" />
       <NativeSystemLibrary Include="zstd" Condition="'$(UseSystemZstd)' == 'true'" />
-      <NativeSystemLibrary Include="rt" Condition="'$(_IsApplePlatform)' != 'true' and '$(_linuxLibcFlavor)' != 'bionic'" />
+      <NativeSystemLibrary Include="rt" Condition="'$(_IsApplePlatform)' != 'true' and '$(_linuxLibcFlavor)' != 'bionic' and '$(_targetOS)' != 'openbsd'" />
       <NativeSystemLibrary Include="log" Condition="'$(_linuxLibcFlavor)' == 'bionic'" />
       <NativeSystemLibrary Include="icucore" Condition="'$(_IsApplePlatform)' == 'true'" />
       <NativeSystemLibrary Include="m" />
@@ -267,6 +269,10 @@ The .NET Foundation licenses this file to you under the MIT license.
       <!-- binskim warning BA3011 The BIND_NOW flag is missing -->
       <LinkerArg Include="-Wl,-z,now" Condition="'$(_IsApplePlatform)' != 'true'" Shim="true" />
       <LinkerArg Include="-Wl,-z,stack-size=$(IlcDefaultStackSize)" Condition="'$(IlcDefaultStackSize)' != ''" Shim="true" />
+      <!-- OpenBSD enforces IBT/BTCFI on amd64. NativeAOT codegen is not endbr64-instrumented, so
+           indirect branches (e.g. switch jump tables) to non-endbr64 targets fault with SIGILL.
+           Opt out of branch-target CFI enforcement via the PT_OPENBSD_NOBTCFI program header. -->
+      <LinkerArg Include="-Wl,-z,nobtcfi" Condition="'$(_targetOS)' == 'openbsd'" />
       <!-- this workaround can be deleted once the minimum supported glibc version
            (runtime's official build machine's glibc version) is at least 2.33
            see https://github.com/bminor/glibc/commit/99468ed45f5a58f584bab60364af937eb6f8afda -->
diff --git a/src/coreclr/nativeaot/CMakeLists.txt b/src/coreclr/nativeaot/CMakeLists.txt
index 083cdbf5a29b71..8fd36f1a78ee27 100644
--- a/src/coreclr/nativeaot/CMakeLists.txt
+++ b/src/coreclr/nativeaot/CMakeLists.txt
@@ -28,6 +28,12 @@ if(CLR_CMAKE_TARGET_ANDROID)
     set(FEATURE_JAVAMARSHAL 1)
 endif()
 
+if(CLR_CMAKE_TARGET_OPENBSD)
+    # OpenBSD's ld.so cannot resolve native TLS relocations in shared objects and has no
+    # __tls_get_addr, so the runtime must use emulated TLS (like Android).
+    add_definitions(-DFEATURE_EMULATED_TLS)
+endif()
+
 if(NOT DEFINED FEATURE_JAVAMARSHAL)
   set(FEATURE_JAVAMARSHAL $<IF:$<CONFIG:Debug,Checked>,1,0>)
 endif()
diff --git a/src/coreclr/nativeaot/Runtime/CMakeLists.txt b/src/coreclr/nativeaot/Runtime/CMakeLists.txt
index e491f54a562445..2b72378c6042cf 100644
--- a/src/coreclr/nativeaot/Runtime/CMakeLists.txt
+++ b/src/coreclr/nativeaot/Runtime/CMakeLists.txt
@@ -288,7 +288,10 @@ if(CLR_CMAKE_TARGET_WIN32)
     add_compile_options($<$<COMPILE_LANGUAGE:ASM_MASM>:/safeseh>)
   endif()
 else()
-  if(NOT CLR_CMAKE_TARGET_APPLE AND NOT CLR_CMAKE_TARGET_ARCH_WASM)
+  # OpenBSD marks the main executable's read-only segments immutable (mimmutable) at load,
+  # so the runtime cannot mprotect the read-only GS cookie page to writable to initialize it.
+  # Keep the cookie in writable memory there, like Apple.
+  if(NOT CLR_CMAKE_TARGET_APPLE AND NOT CLR_CMAKE_TARGET_ARCH_WASM AND NOT CLR_CMAKE_TARGET_OPENBSD)
     add_definitions(-DFEATURE_READONLY_GS_COOKIE)
   endif()
   include(unix/configure.cmake)
diff --git a/src/coreclr/nativeaot/Runtime/unix/NativeContext.cpp b/src/coreclr/nativeaot/Runtime/unix/NativeContext.cpp
index d1a6b300054622..d5fa9904b90ad3 100644
--- a/src/coreclr/nativeaot/Runtime/unix/NativeContext.cpp
+++ b/src/coreclr/nativeaot/Runtime/unix/NativeContext.cpp
@@ -460,6 +460,29 @@
 #define MCREG_T6(mc)      ((mc).regs[31])
 #define MCREG_Pc(mc)      ((mc).pc)
 
+#elif defined(__OpenBSD__)
+
+// On OpenBSD, ucontext_t is an alias for struct sigcontext and the registers
+// are stored directly in it; there is no uc_mcontext sub-structure. The mc
+// argument here is the struct sigcontext (i.e. the ucontext) itself.
+#define MCREG_Rip(mc)       ((mc).sc_rip)
+#define MCREG_Rsp(mc)       ((mc).sc_rsp)
+#define MCREG_Rax(mc)       ((mc).sc_rax)
+#define MCREG_Rbx(mc)       ((mc).sc_rbx)
+#define MCREG_Rcx(mc)       ((mc).sc_rcx)
+#define MCREG_Rdx(mc)       ((mc).sc_rdx)
+#define MCREG_Rsi(mc)       ((mc).sc_rsi)
+#define MCREG_Rdi(mc)       ((mc).sc_rdi)
+#define MCREG_Rbp(mc)       ((mc).sc_rbp)
+#define MCREG_R8(mc)        ((mc).sc_r8)
+#define MCREG_R9(mc)        ((mc).sc_r9)
+#define MCREG_R10(mc)       ((mc).sc_r10)
+#define MCREG_R11(mc)       ((mc).sc_r11)
+#define MCREG_R12(mc)       ((mc).sc_r12)
+#define MCREG_R13(mc)       ((mc).sc_r13)
+#define MCREG_R14(mc)       ((mc).sc_r14)
+#define MCREG_R15(mc)       ((mc).sc_r15)
+
 #else
 
 // For FreeBSD, as found in x86/ucontext.h
@@ -526,6 +549,16 @@
 
 #endif // __APPLE__
 
+// Accessors used to obtain the structure holding the machine registers from a
+// ucontext_t. On most platforms the registers live in the uc_mcontext member,
+// but on OpenBSD ucontext_t is an alias for struct sigcontext and the registers
+// are stored directly in it.
+#if defined(__OpenBSD__)
+#define MCONTEXT_FROM_NATIVE(nativeContext) (*(nativeContext))
+#else
+#define MCONTEXT_FROM_NATIVE(nativeContext) ((nativeContext)->uc_mcontext)
+#endif
+
 #if defined(HOST_AMD64)
 #define ASSIGN_CONTROL_REGS \
     ASSIGN_REG(Rip, IP)     \
@@ -540,8 +573,8 @@
     ASSIGN_REG(R15, R15)
 
 #define ASSIGN_TWO_ARGUMENT_REGS(arg0Reg, arg1Reg)    \
-    MCREG_Rdi(nativeContext->uc_mcontext) = arg0Reg;  \
-    MCREG_Rsi(nativeContext->uc_mcontext) = arg1Reg;
+    MCREG_Rdi(MCONTEXT_FROM_NATIVE(nativeContext)) = arg0Reg;  \
+    MCREG_Rsi(MCONTEXT_FROM_NATIVE(nativeContext)) = arg1Reg;
 
 #elif defined(HOST_X86)
 #define ASSIGN_CONTROL_REGS \
@@ -662,7 +695,7 @@
 void NativeContextToPalContext(const void* context, PAL_LIMITED_CONTEXT* palContext)
 {
     ucontext_t *nativeContext = (ucontext_t*)context;
-#define ASSIGN_REG(regNative, regPal) palContext->regPal = MCREG_##regNative(nativeContext->uc_mcontext);
+#define ASSIGN_REG(regNative, regPal) palContext->regPal = MCREG_##regNative(MCONTEXT_FROM_NATIVE(nativeContext));
     ASSIGN_CONTROL_REGS
     ASSIGN_INTEGER_REGS
 #undef ASSIGN_REG
@@ -673,7 +706,7 @@ void RedirectNativeContext(void* context, const PAL_LIMITED_CONTEXT* palContext,
 {
     ucontext_t *nativeContext = (ucontext_t*)context;
 
-#define ASSIGN_REG(regNative, regPal) MCREG_##regNative(nativeContext->uc_mcontext) = palContext->regPal;
+#define ASSIGN_REG(regNative, regPal) MCREG_##regNative(MCONTEXT_FROM_NATIVE(nativeContext)) = palContext->regPal;
     ASSIGN_CONTROL_REGS
 #undef ASSIGN_REG
     ASSIGN_TWO_ARGUMENT_REGS(arg0Reg, arg1Reg);
@@ -694,37 +727,37 @@ uint64_t GetRegisterValueByIndex(void* context, uint32_t index)
     switch (index)
     {
         case 0:
-            return MCREG_Rax(nativeContext->uc_mcontext);
+            return MCREG_Rax(MCONTEXT_FROM_NATIVE(nativeContext));
         case 1:
-            return MCREG_Rcx(nativeContext->uc_mcontext);
+            return MCREG_Rcx(MCONTEXT_FROM_NATIVE(nativeContext));
         case 2:
-            return MCREG_Rdx(nativeContext->uc_mcontext);
+            return MCREG_Rdx(MCONTEXT_FROM_NATIVE(nativeContext));
         case 3:
-            return MCREG_Rbx(nativeContext->uc_mcontext);
+            return MCREG_Rbx(MCONTEXT_FROM_NATIVE(nativeContext));
         case 4:
-            return MCREG_Rsp(nativeContext->uc_mcontext);
+            return MCREG_Rsp(MCONTEXT_FROM_NATIVE(nativeContext));
         case 5:
-            return MCREG_Rbp(nativeContext->uc_mcontext);
+            return MCREG_Rbp(MCONTEXT_FROM_NATIVE(nativeContext));
         case 6:
-            return MCREG_Rsi(nativeContext->uc_mcontext);
+            return MCREG_Rsi(MCONTEXT_FROM_NATIVE(nativeContext));
         case 7:
-            return MCREG_Rdi(nativeContext->uc_mcontext);
+            return MCREG_Rdi(MCONTEXT_FROM_NATIVE(nativeContext));
         case 8:
-            return MCREG_R8(nativeContext->uc_mcontext);
+            return MCREG_R8(MCONTEXT_FROM_NATIVE(nativeContext));
         case 9:
-            return MCREG_R9(nativeContext->uc_mcontext);
+            return MCREG_R9(MCONTEXT_FROM_NATIVE(nativeContext));
         case 10:
-            return MCREG_R10(nativeContext->uc_mcontext);
+            return MCREG_R10(MCONTEXT_FROM_NATIVE(nativeContext));
         case 11:
-            return MCREG_R11(nativeContext->uc_mcontext);
+            return MCREG_R11(MCONTEXT_FROM_NATIVE(nativeContext));
         case 12:
-            return MCREG_R12(nativeContext->uc_mcontext);
+            return MCREG_R12(MCONTEXT_FROM_NATIVE(nativeContext));
         case 13:
-            return MCREG_R13(nativeContext->uc_mcontext);
+            return MCREG_R13(MCONTEXT_FROM_NATIVE(nativeContext));
         case 14:
-            return MCREG_R14(nativeContext->uc_mcontext);
+            return MCREG_R14(MCONTEXT_FROM_NATIVE(nativeContext));
         case 15:
-            return MCREG_R15(nativeContext->uc_mcontext);
+            return MCREG_R15(MCONTEXT_FROM_NATIVE(nativeContext));
     }
 
     ASSERT(false);
@@ -735,7 +768,7 @@ uint64_t GetRegisterValueByIndex(void* context, uint32_t index)
 uint64_t GetPC(void* context)
 {
     ucontext_t *nativeContext = (ucontext_t*)context;
-    return MCREG_Rip(nativeContext->uc_mcontext);
+    return MCREG_Rip(MCONTEXT_FROM_NATIVE(nativeContext));
 }
 
 #endif // HOST_AMD64
@@ -777,23 +810,23 @@ uint64_t GetPC(void* context)
     uint64_t& NATIVE_CONTEXT::Pc() { return (uint64_t&)MCREG_Pc(ctx.uc_mcontext); }
 
 #elif defined(TARGET_AMD64)
-    uint64_t& NATIVE_CONTEXT::Rax(){ return (uint64_t&)MCREG_Rax(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rcx(){ return (uint64_t&)MCREG_Rcx(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rdx(){ return (uint64_t&)MCREG_Rdx(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rbx(){ return (uint64_t&)MCREG_Rbx(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rsp(){ return (uint64_t&)MCREG_Rsp(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rbp(){ return (uint64_t&)MCREG_Rbp(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rsi(){ return (uint64_t&)MCREG_Rsi(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rdi(){ return (uint64_t&)MCREG_Rdi(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R8(){ return (uint64_t&)MCREG_R8(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R9(){ return (uint64_t&)MCREG_R9(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R10(){ return (uint64_t&)MCREG_R10(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R11(){ return (uint64_t&)MCREG_R11(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R12(){ return (uint64_t&)MCREG_R12(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R13(){ return (uint64_t&)MCREG_R13(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R14(){ return (uint64_t&)MCREG_R14(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::R15(){ return (uint64_t&)MCREG_R15(ctx.uc_mcontext); }
-    uint64_t& NATIVE_CONTEXT::Rip(){ return (uint64_t&)MCREG_Rip(ctx.uc_mcontext); }
+    uint64_t& NATIVE_CONTEXT::Rax(){ return (uint64_t&)MCREG_Rax(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rcx(){ return (uint64_t&)MCREG_Rcx(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rdx(){ return (uint64_t&)MCREG_Rdx(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rbx(){ return (uint64_t&)MCREG_Rbx(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rsp(){ return (uint64_t&)MCREG_Rsp(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rbp(){ return (uint64_t&)MCREG_Rbp(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rsi(){ return (uint64_t&)MCREG_Rsi(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rdi(){ return (uint64_t&)MCREG_Rdi(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R8(){ return (uint64_t&)MCREG_R8(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R9(){ return (uint64_t&)MCREG_R9(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R10(){ return (uint64_t&)MCREG_R10(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R11(){ return (uint64_t&)MCREG_R11(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R12(){ return (uint64_t&)MCREG_R12(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R13(){ return (uint64_t&)MCREG_R13(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R14(){ return (uint64_t&)MCREG_R14(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::R15(){ return (uint64_t&)MCREG_R15(MCONTEXT_FROM_NATIVE(&ctx)); }
+    uint64_t& NATIVE_CONTEXT::Rip(){ return (uint64_t&)MCREG_Rip(MCONTEXT_FROM_NATIVE(&ctx)); }
 
 #elif defined(TARGET_ARM)
     uint64_t& NATIVE_CONTEXT::Pc(){ return (uint64_t&)MCREG_Pc(ctx.uc_mcontext); }
diff --git a/src/coreclr/nativeaot/Runtime/unix/PalUnix.cpp b/src/coreclr/nativeaot/Runtime/unix/PalUnix.cpp
index 52aaec80d74eab..c6fa26c221a210 100644
--- a/src/coreclr/nativeaot/Runtime/unix/PalUnix.cpp
+++ b/src/coreclr/nativeaot/Runtime/unix/PalUnix.cpp
@@ -54,6 +54,10 @@
 #include <pthread_np.h>
 #endif
 
+#if defined(__OpenBSD__)
+#include <pthread_np.h>
+#endif
+
 #if HAVE_LWP_SELF
 #include <lwp.h>
 #endif
@@ -1193,6 +1197,15 @@ bool PalGetMaximumStackBounds(_Out_ void** ppStackLowOut, _Out_ void** ppStackHi
     // This is a Mac specific method
     pStackHighOut = pthread_get_stackaddr_np(pthread_self());
     pStackLowOut = ((uint8_t *)pStackHighOut - pthread_get_stacksize_np(pthread_self()));
+#elif defined(__OpenBSD__)
+    // OpenBSD provides the stack segment of the current thread via pthread_stackseg_np.
+    // ss_sp points to the top (highest address) of the stack.
+    stack_t stack;
+    int status = pthread_stackseg_np(pthread_self(), &stack);
+    ASSERT_MSG(status == 0, "pthread_stackseg_np call failed");
+
+    pStackHighOut = stack.ss_sp;
+    pStackLowOut = (uint8_t*)stack.ss_sp - stack.ss_size;
 #else // __APPLE__
     pthread_attr_t attr;
     size_t stackSize;
diff --git a/src/coreclr/pal/inc/pal.h b/src/coreclr/pal/inc/pal.h
index b724fbc9dede76..2abfc57d0d3f3e 100644
--- a/src/coreclr/pal/inc/pal.h
+++ b/src/coreclr/pal/inc/pal.h
@@ -3513,7 +3513,15 @@ PAL_FreeExceptionRecords(
 #define EXCEPTION_EXECUTE_HANDLER   1
 #define EXCEPTION_CONTINUE_EXECUTION -1
 
-struct PAL_SEHException
+struct
+#ifdef __OpenBSD__
+// OpenBSD's libc++ compares exception type_info by pointer (relying on ld.so to merge
+// type_info across shared objects) instead of falling back to a name compare like Linux's
+// libstdc++. Default visibility lets ld.so merge type_info into one instance so a
+// PAL_SEHException thrown in one PAL DSO is caught in another (e.g. the pal_sxs test).
+__attribute__((visibility("default")))
+#endif
+PAL_SEHException
 {
 private:
     static const SIZE_T NoTargetFrameSp = (SIZE_T)SIZE_MAX;
diff --git a/src/coreclr/pal/inc/unixasmmacrosamd64.inc b/src/coreclr/pal/inc/unixasmmacrosamd64.inc
index 90c8947e754297..819ef29ba84257 100644
--- a/src/coreclr/pal/inc/unixasmmacrosamd64.inc
+++ b/src/coreclr/pal/inc/unixasmmacrosamd64.inc
@@ -430,6 +430,12 @@ C_FUNC(\Name\()_End):
 
 .macro INLINE_GETTHREAD
         // Inlined version of call C_FUNC(RhpGetThread)
+#ifdef FEATURE_EMULATED_TLS
+        // Emulated TLS (e.g. OpenBSD, which has no __tls_get_addr): call a C helper instead
+        // of inlining the native thread-local access.
+        call    C_FUNC(GetThreadHelper)
+#else
         INLINE_GET_TLS_VAR t_CurrentThreadInfo
         mov rax, [rax + OFFSETOF__ThreadLocalInfo__m_pThread]
+#endif
 .endm
diff --git a/src/coreclr/pal/src/arch/amd64/signalhandlerhelper.cpp b/src/coreclr/pal/src/arch/amd64/signalhandlerhelper.cpp
index 3f70c41d1ef93c..f4c62702865419 100644
--- a/src/coreclr/pal/src/arch/amd64/signalhandlerhelper.cpp
+++ b/src/coreclr/pal/src/arch/amd64/signalhandlerhelper.cpp
@@ -30,7 +30,7 @@ Parameters :
 void ExecuteHandlerOnCustomStack(int code, siginfo_t *siginfo, void *context, size_t customSp, SignalHandlerWorkerReturnPoint* returnPoint)
 {
     ucontext_t *ucontext = (ucontext_t *)context;
-    size_t faultSp = (size_t)MCREG_Rsp(ucontext->uc_mcontext);
+    size_t faultSp = (size_t)MCREG_Rsp(MCONTEXT_FROM_NATIVE(ucontext));
 
     _ASSERTE(IS_ALIGNED(faultSp, 8));
 
@@ -54,8 +54,8 @@ void ExecuteHandlerOnCustomStack(int code, siginfo_t *siginfo, void *context, si
     size_t* sp = (size_t*)customSp;
 
     // Build fake stack frame to enable the stack unwinder to unwind from signal_handler_worker to the faulting instruction
-    *--sp = (size_t)MCREG_Rip(ucontext->uc_mcontext);
-    *--sp = (size_t)MCREG_Rbp(ucontext->uc_mcontext);
+    *--sp = (size_t)MCREG_Rip(MCONTEXT_FROM_NATIVE(ucontext));
+    *--sp = (size_t)MCREG_Rbp(MCONTEXT_FROM_NATIVE(ucontext));
     size_t fp = (size_t)sp;
     *--sp = fakeFrameReturnAddress;
 
diff --git a/src/coreclr/pal/src/exception/signal.cpp b/src/coreclr/pal/src/exception/signal.cpp
index c5b6886b562219..79e5629e48c41b 100644
--- a/src/coreclr/pal/src/exception/signal.cpp
+++ b/src/coreclr/pal/src/exception/signal.cpp
@@ -2,19 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 /*++
-
-
-
 Module Name:
-
     exception/signal.cpp
 
 Abstract:
-
     Signal handler implementation (map signals to exceptions)
-
-
-
 --*/
 
 #include "pal/dbgmsg.h"
@@ -379,7 +371,15 @@ bool IsRunningOnAlternateStack(void *context)
     {
         // Note: WSL doesn't return the alternate signal ranges in the uc_stack (the whole structure is zeroed no
         // matter whether the code is running on an alternate stack or not). So the check would always fail on WSL.
+#ifdef TARGET_OPENBSD
+        // OpenBSD's ucontext_t (struct sigcontext) doesn't carry the signal stack,
+        // so query the currently installed alternate stack directly.
+        stack_t signalStackStorage;
+        stack_t *signalStack = &signalStackStorage;
+        sigaltstack(NULL, signalStack);
+#else
         stack_t *signalStack = &((native_context_t *)context)->uc_stack;
+#endif
         // Check if the signalStack local variable address is within the alternate stack range. If it is not,
         // then either the alternate stack was not installed at all or the current method is not running on it.
         void* alternateStackEnd = (char *)signalStack->ss_sp + signalStack->ss_size;
diff --git a/src/coreclr/pal/src/file/file.cpp b/src/coreclr/pal/src/file/file.cpp
index b09b3d56d024e3..2ee393980c9dba 100644
--- a/src/coreclr/pal/src/file/file.cpp
+++ b/src/coreclr/pal/src/file/file.cpp
@@ -420,6 +420,7 @@ CorUnix::InternalCreateFile(
     int   filed = -1;
     int   create_flags = (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
     int   open_flags = 0;
+    int   open_flags_for_syscall = 0;
 
     // track whether we've created the file with the intended name,
     // so that it can be removed on failure exit
@@ -575,7 +576,22 @@ CorUnix::InternalCreateFile(
         goto done;
     }
 
-    filed = InternalOpen(lpUnixPath, open_flags, create_flags);
+
+    open_flags_for_syscall = open_flags;
+#if defined(__OpenBSD__)
+    /* On OpenBSD, open() requires O_RDWR or O_WRONLY to be specified along with
+       O_TRUNC, otherwise it fails with EINVAL. Windows allows truncating a file
+       that is opened for read-only access (e.g. GENERIC_READ with CREATE_ALWAYS).
+       Upgrade only the flags passed to open() so the truncation succeeds, while
+       leaving open_flags (used later for access checks such as file mappings)
+       unchanged to preserve the caller's requested access. */
+    if ((open_flags_for_syscall & O_TRUNC) && (open_flags_for_syscall & O_ACCMODE) == O_RDONLY)
+    {
+        open_flags_for_syscall = (open_flags_for_syscall & ~O_ACCMODE) | O_RDWR;
+    }
+#endif // __OpenBSD__
+
+    filed = InternalOpen(lpUnixPath, open_flags_for_syscall, create_flags);
     TRACE("Allocated file descriptor [%d]\n", filed);
 
     if ( filed < 0 )
diff --git a/src/coreclr/pal/src/include/pal/context.h b/src/coreclr/pal/src/include/pal/context.h
index 9d5e3f01e5f59c..04829d95a17ccf 100644
--- a/src/coreclr/pal/src/include/pal/context.h
+++ b/src/coreclr/pal/src/include/pal/context.h
@@ -2,19 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 /*++
-
-
-
 Module Name:
-
     include/pal/context.h
 
 Abstract:
-
     Header file for thread context utility functions.
-
-
-
 --*/
 
 #ifndef _PAL_CONTEXT_H_
@@ -42,6 +34,15 @@ typedef ucontext_t native_context_t;
 #error Native context type is not known on this platform!
 #endif  // HAVE_UCONTEXT_T
 
+// Helper to obtain the machine-register container from a native_context_t pointer.
+// On most platforms the registers live in the uc_mcontext sub-structure. On OpenBSD
+// native_context_t is an alias for struct sigcontext and holds the registers directly.
+#ifdef TARGET_OPENBSD
+#define MCONTEXT_FROM_NATIVE(ucontextPtr)  (*(ucontextPtr))
+#else
+#define MCONTEXT_FROM_NATIVE(ucontextPtr)  ((ucontextPtr)->uc_mcontext)
+#endif
+
 #if !HAVE_MACH_EXCEPTIONS
 
 #if defined(XSTATE_SUPPORTED) && defined(HOST_AMD64) && !HAVE_PUBLIC_XSTATE_STRUCT
@@ -1007,6 +1008,44 @@ inline void *FPREG_Xstate_Hi16Zmm(const ucontext_t *uc, uint32_t *featureSize)
 
 #define FPREG_Xmm(uc, index)    *(M128A*) &(FPSTATE(uc).fp_fxsave.xmm[index])
 #define FPREG_St(uc, index)     *(M128A*) &(FPSTATE(uc).fp_fxsave.fp[index].value)
+#elif defined(TARGET_OPENBSD)
+
+    // On OpenBSD, ucontext_t is an alias for struct sigcontext and the registers
+    // are stored directly in it (accessed here via MCONTEXT_FROM_NATIVE).
+#define MCREG_Rbp(mc)       ((mc).sc_rbp)
+#define MCREG_Rip(mc)       ((mc).sc_rip)
+#define MCREG_Rsp(mc)       ((mc).sc_rsp)
+#define MCREG_Rsi(mc)       ((mc).sc_rsi)
+#define MCREG_Rdi(mc)       ((mc).sc_rdi)
+#define MCREG_Rbx(mc)       ((mc).sc_rbx)
+#define MCREG_Rdx(mc)       ((mc).sc_rdx)
+#define MCREG_Rcx(mc)       ((mc).sc_rcx)
+#define MCREG_Rax(mc)       ((mc).sc_rax)
+#define MCREG_R8(mc)        ((mc).sc_r8)
+#define MCREG_R9(mc)        ((mc).sc_r9)
+#define MCREG_R10(mc)       ((mc).sc_r10)
+#define MCREG_R11(mc)       ((mc).sc_r11)
+#define MCREG_R12(mc)       ((mc).sc_r12)
+#define MCREG_R13(mc)       ((mc).sc_r13)
+#define MCREG_R14(mc)       ((mc).sc_r14)
+#define MCREG_R15(mc)       ((mc).sc_r15)
+#define MCREG_EFlags(mc)    ((mc).sc_rflags)
+#define MCREG_SegCs(mc)     ((mc).sc_cs)
+
+  // from machine/fpu.h: struct fxsave64, referenced via sigcontext::sc_fpstate
+#define FPSTATE(uc)             ((struct fxsave64*)((uc)->sc_fpstate))
+#define FPREG_ControlWord(uc)   FPSTATE(uc)->fx_fcw
+#define FPREG_StatusWord(uc)    FPSTATE(uc)->fx_fsw
+#define FPREG_TagWord(uc)       FPSTATE(uc)->fx_ftw
+#define FPREG_MxCsr(uc)         FPSTATE(uc)->fx_mxcsr
+#define FPREG_MxCsr_Mask(uc)    FPSTATE(uc)->fx_mxcsr_mask
+#define FPREG_ErrorOffset(uc)   *(DWORD*) &(FPSTATE(uc)->fx_rip)
+#define FPREG_ErrorSelector(uc) *((WORD*) &(FPSTATE(uc)->fx_rip) + 2)
+#define FPREG_DataOffset(uc)    *(DWORD*) &(FPSTATE(uc)->fx_rdp)
+#define FPREG_DataSelector(uc)  *((WORD*) &(FPSTATE(uc)->fx_rdp) + 2)
+
+#define FPREG_Xmm(uc, index)    *(M128A*) &(FPSTATE(uc)->fx_xmm[index])
+#define FPREG_St(uc, index)     *(M128A*) &(FPSTATE(uc)->fx_st[index])
 #else //__APPLE__
 
     // For FreeBSD, as found in x86/ucontext.h
diff --git a/src/coreclr/pal/src/safecrt/vsprintf.cpp b/src/coreclr/pal/src/safecrt/vsprintf.cpp
index 360222d5dc6798..25cd3766591c35 100644
--- a/src/coreclr/pal/src/safecrt/vsprintf.cpp
+++ b/src/coreclr/pal/src/safecrt/vsprintf.cpp
@@ -23,6 +23,78 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#if defined(__OpenBSD__)
+// OpenBSD's libc doesn't support the legacy %C/%S wide specifiers (glibc and the
+// other BSDs do). Rewrite them to %lc/%ls. Returns a malloc'd copy, or NULL on
+// failure (callers fall back to the original format).
+static char *PAL_TranslateWideFormatForOpenBSD(const char *format)
+{
+    size_t length = strlen(format);
+    // Each rewrite adds one 'l', so doubling is always enough.
+    char *translated = (char *)malloc(length * 2 + 1);
+    if (translated == NULL)
+    {
+        return NULL;
+    }
+
+    const char *in = format;
+    char *out = translated;
+    while (*in != '\0')
+    {
+        if (*in != '%')
+        {
+            *out++ = *in++;
+            continue;
+        }
+
+        *out++ = *in++;
+        if (*in == '%')
+        {
+            *out++ = *in++; // literal "%%"
+            continue;
+        }
+
+        // Skip flags, width, precision and length modifiers.
+        while (*in != '\0' && strchr("-+ #0123456789.*hlLwIqjzt", *in) != NULL)
+        {
+            *out++ = *in++;
+        }
+
+        if (*in == 'C')
+        {
+            *out++ = 'l';
+            *out++ = 'c';
+            in++;
+        }
+        else if (*in == 'S')
+        {
+            *out++ = 'l';
+            *out++ = 's';
+            in++;
+        }
+        else if (*in != '\0')
+        {
+            *out++ = *in++;
+        }
+    }
+    *out = '\0';
+
+    return translated;
+}
+
+static int PAL_safecrt_vsnprintf(char *string, size_t count, const char *format, va_list ap)
+{
+    char *translated = PAL_TranslateWideFormatForOpenBSD(format);
+    int retvalue = vsnprintf(string, count, translated != NULL ? translated : format, ap);
+    free(translated);
+    return retvalue;
+}
+
+#define PAL_SAFECRT_VSNPRINTF PAL_safecrt_vsnprintf
+#else // !defined(__OpenBSD__)
+#define PAL_SAFECRT_VSNPRINTF vsnprintf
+#endif // defined(__OpenBSD__)
+
 DLLEXPORT int __cdecl vsprintf_s (
         char *string,
         size_t sizeInBytes,
@@ -36,7 +108,7 @@ DLLEXPORT int __cdecl vsprintf_s (
     _VALIDATE_RETURN(format != NULL, EINVAL, -1);
     _VALIDATE_RETURN(string != NULL && sizeInBytes > 0, EINVAL, -1);
 
-    retvalue = vsnprintf(string, sizeInBytes, format, ap);
+    retvalue = PAL_SAFECRT_VSNPRINTF(string, sizeInBytes, format, ap);
     if (retvalue < 0)
     {
         string[0] = '\0';
@@ -77,7 +149,7 @@ DLLEXPORT int __cdecl _vsnprintf_s (
     if (sizeInBytes > count)
     {
         save_errno = errno;
-        retvalue = vsnprintf(string, count + 1, format, ap);
+        retvalue = PAL_SAFECRT_VSNPRINTF(string, count + 1, format, ap);
         if (retvalue > (int)(count + 1))
         {
             /* the string has been truncated, return -1 */
@@ -92,7 +164,7 @@ DLLEXPORT int __cdecl _vsnprintf_s (
     else /* sizeInBytes <= count */
     {
         save_errno = errno;
-        retvalue = vsnprintf(string, sizeInBytes, format, ap);
+        retvalue = PAL_SAFECRT_VSNPRINTF(string, sizeInBytes, format, ap);
         string[sizeInBytes - 1] = '\0';
         /* we allow truncation if count == _TRUNCATE */
         if (retvalue >= (int)sizeInBytes && count == _TRUNCATE)
diff --git a/src/coreclr/pal/src/thread/context.cpp b/src/coreclr/pal/src/thread/context.cpp
index 16964ce4e885ed..259cfb320b174a 100644
--- a/src/coreclr/pal/src/thread/context.cpp
+++ b/src/coreclr/pal/src/thread/context.cpp
@@ -55,6 +55,10 @@ typedef int __ptrace_request;
 #include <machine/npx.h>
 #endif  // HAVE_MACHINE_NPX_H
 
+#ifdef __OpenBSD__
+#include <sys/ptrace.h>
+#endif  // __OpenBSD__
+
 #if HAVE_PT_REGS
 #include <asm/ptrace.h>
 #endif  // HAVE_PT_REGS
@@ -499,7 +503,7 @@ BOOL CONTEXT_GetRegisters(DWORD processId, LPCONTEXT lpContext)
 #if HAVE_PT_REGS
 #define ASSIGN_REG(reg) MCREG_##reg(registers.uc_mcontext) = PTREG_##reg(ptrace_registers);
 #elif HAVE_BSD_REGS_T
-#define ASSIGN_REG(reg) MCREG_##reg(registers.uc_mcontext) = BSDREG_##reg(ptrace_registers);
+#define ASSIGN_REG(reg) MCREG_##reg(MCONTEXT_FROM_NATIVE(&registers)) = BSDREG_##reg(ptrace_registers);
 #else
 #define ASSIGN_REG(reg)
 	ASSERT("Don't know how to get the context of another process on this platform!");
@@ -701,7 +705,7 @@ Return value :
 --*/
 void CONTEXTToNativeContext(CONST CONTEXT *lpContext, native_context_t *native)
 {
-#define ASSIGN_REG(reg) MCREG_##reg(native->uc_mcontext) = lpContext->reg;
+#define ASSIGN_REG(reg) MCREG_##reg(MCONTEXT_FROM_NATIVE(native)) = lpContext->reg;
     if ((lpContext->ContextFlags & CONTEXT_CONTROL) == CONTEXT_CONTROL)
     {
         ASSIGN_CONTROL_REGS
@@ -1043,7 +1047,7 @@ void CONTEXTFromNativeContext(const native_context_t *native, LPCONTEXT lpContex
 {
     lpContext->ContextFlags = contextFlags;
 
-#define ASSIGN_REG(reg) lpContext->reg = MCREG_##reg(native->uc_mcontext);
+#define ASSIGN_REG(reg) lpContext->reg = MCREG_##reg(MCONTEXT_FROM_NATIVE(native));
     if ((contextFlags & CONTEXT_CONTROL) == CONTEXT_CONTROL)
     {
         ASSIGN_CONTROL_REGS
@@ -1317,7 +1321,7 @@ Return value :
 LPVOID GetNativeContextPC(const native_context_t *context)
 {
 #ifdef HOST_AMD64
-    return (LPVOID)MCREG_Rip(context->uc_mcontext);
+    return (LPVOID)MCREG_Rip(MCONTEXT_FROM_NATIVE(context));
 #elif defined(HOST_X86)
     return (LPVOID) MCREG_Eip(context->uc_mcontext);
 #elif defined(HOST_S390X)
@@ -1345,7 +1349,7 @@ Return value :
 LPVOID GetNativeContextSP(const native_context_t *context)
 {
 #ifdef HOST_AMD64
-    return (LPVOID)MCREG_Rsp(context->uc_mcontext);
+    return (LPVOID)MCREG_Rsp(MCONTEXT_FROM_NATIVE(context));
 #elif defined(HOST_X86)
     return (LPVOID) MCREG_Esp(context->uc_mcontext);
 #elif defined(HOST_S390X)
diff --git a/src/coreclr/pal/src/thread/thread.cpp b/src/coreclr/pal/src/thread/thread.cpp
index f7174e69ba5be0..5460b784aa09df 100644
--- a/src/coreclr/pal/src/thread/thread.cpp
+++ b/src/coreclr/pal/src/thread/thread.cpp
@@ -2372,7 +2372,9 @@ CPalThread::GetStackLimit()
     int status = pthread_stackseg_np(pthread_self(), &stack);
     _ASSERT_MSG(status == 0, "pthread_stackseg_np call failed");
 
-    stackLimit = (void*)stack.ss_size;
+    // ss_sp is the stack base (highest address) and ss_size is the size, so the
+    // limit (lowest address) is ss_sp - ss_size. The stack grows down from ss_sp.
+    stackLimit = (void*)((size_t)stack.ss_sp - stack.ss_size);
 #else
     pthread_attr_t attr;
     size_t stackSize;
diff --git a/src/coreclr/pal/tests/palsuite/CMakeLists.txt b/src/coreclr/pal/tests/palsuite/CMakeLists.txt
index b80c283430f6e2..b52e743d5e3ddd 100644
--- a/src/coreclr/pal/tests/palsuite/CMakeLists.txt
+++ b/src/coreclr/pal/tests/palsuite/CMakeLists.txt
@@ -400,6 +400,14 @@ target_link_libraries(paltests
   ${COMMON_TEST_LIBRARIES}
 )
 
+if(CLR_CMAKE_TARGET_OPENBSD)
+  # A few PAL tests (e.g. VirtualAlloc/VirtualProtect) deliberately request simultaneous
+  # W+X (PAGE_EXECUTE_READWRITE) mappings to exercise the PAL API. OpenBSD rejects those
+  # unless the binary carries the PT_OPENBSD_WXNEEDED note and runs from a wxallowed mount.
+  # The runtime itself never needs this (it uses W^X double-mapping), so scope it here.
+  target_link_options(paltests PRIVATE "-Wl,-z,wxneeded")
+endif(CLR_CMAKE_TARGET_OPENBSD)
+
 install (TARGETS paltests DESTINATION paltests COMPONENT paltests EXCLUDE_FROM_ALL)
 add_dependencies(paltests_install paltests)
 install (PROGRAMS runpaltests.sh runpaltestshelix.sh DESTINATION paltests COMPONENT paltests EXCLUDE_FROM_ALL)
diff --git a/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/CMakeLists.txt b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/CMakeLists.txt
index cda9df41a65e09..94bb721965e43b 100644
--- a/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/CMakeLists.txt
+++ b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/CMakeLists.txt
@@ -4,6 +4,14 @@ if(CLR_CMAKE_HOST_UNIX)
     add_definitions(-DFEATURE_ENABLE_HARDWARE_EXCEPTIONS)
 endif(CLR_CMAKE_HOST_UNIX)
 
+# OpenBSD's libc++ compares exception type_info by pointer and relies on ld.so merging it
+# across DSOs. Each side-by-side DSO statically links the PAL, so export PAL_SEHException's
+# type_info (default visibility, see pal.h) from both via an extra version script while the
+# generated one keeps the rest of the PAL private.
+if(CLR_CMAKE_TARGET_OPENBSD)
+    set(SXS_EXPORT_EXCEPTION_TYPEINFO -Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/openbsd_exception_typeinfo.exports)
+endif(CLR_CMAKE_TARGET_OPENBSD)
+
 # Set the RPATH of paltest_pal_sxs_test1 so that it can find dependencies without needing to set LD_LIBRARY
 # For more information: http://www.cmake.org/Wiki/CMake_RPATH_handling.
 if(CORECLR_SET_RPATH)
@@ -32,6 +40,7 @@ set(DLL1SOURCES dlltest1.cpp)
 add_library_clr(paltest_pal_sxs_test1_dll1 SHARED EXCLUDE_FROM_ALL ${DLL1SOURCES})
 add_custom_target(dlltest1_exports DEPENDS ${EXPORTS_FILE1})
 set_property(TARGET paltest_pal_sxs_test1_dll1 APPEND_STRING PROPERTY LINK_FLAGS ${EXPORTS_LINKER_OPTION1})
+set_property(TARGET paltest_pal_sxs_test1_dll1 APPEND_STRING PROPERTY LINK_FLAGS " ${SXS_EXPORT_EXCEPTION_TYPEINFO}")
 set_property(TARGET paltest_pal_sxs_test1_dll1 APPEND_STRING PROPERTY LINK_DEPENDS ${EXPORTS_FILE1})
 
 add_dependencies(paltest_pal_sxs_test1_dll1
@@ -65,6 +74,7 @@ set(DLL2SOURCES dlltest2.cpp)
 add_library_clr(paltest_pal_sxs_test1_dll2 SHARED EXCLUDE_FROM_ALL ${DLL2SOURCES})
 add_custom_target(dlltest2_exports DEPENDS ${EXPORTS_FILE2})
 set_property(TARGET paltest_pal_sxs_test1_dll2 APPEND_STRING PROPERTY LINK_FLAGS ${EXPORTS_LINKER_OPTION2})
+set_property(TARGET paltest_pal_sxs_test1_dll2 APPEND_STRING PROPERTY LINK_FLAGS " ${SXS_EXPORT_EXCEPTION_TYPEINFO}")
 set_property(TARGET paltest_pal_sxs_test1_dll2 APPEND_STRING PROPERTY LINK_DEPENDS ${EXPORTS_FILE2})
 
 add_dependencies(paltest_pal_sxs_test1_dll2
diff --git a/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/exceptionsxs.cpp b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/exceptionsxs.cpp
index c538f52235a6f8..d63993f16f3e94 100644
--- a/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/exceptionsxs.cpp
+++ b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/exceptionsxs.cpp
@@ -18,7 +18,7 @@
 #include <sys/utsname.h>
 #include <unistd.h>
 
-#ifndef __HAIKU__
+#if !defined(__HAIKU__) && !defined(__OpenBSD__)
 #include <sys/ucontext.h>
 #endif
 
diff --git a/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/openbsd_exception_typeinfo.exports b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/openbsd_exception_typeinfo.exports
new file mode 100644
index 00000000000000..28f39db2d00b73
--- /dev/null
+++ b/src/coreclr/pal/tests/palsuite/exception_handling/pal_sxs/test1/openbsd_exception_typeinfo.exports
@@ -0,0 +1,5 @@
+{
+    global:
+        _ZTI16PAL_SEHException;
+        _ZTS16PAL_SEHException;
+};
diff --git a/src/coreclr/tools/Common/CommandLineHelpers.cs b/src/coreclr/tools/Common/CommandLineHelpers.cs
index d13d782327467b..58ff170a7e5252 100644
--- a/src/coreclr/tools/Common/CommandLineHelpers.cs
+++ b/src/coreclr/tools/Common/CommandLineHelpers.cs
@@ -25,7 +25,7 @@ internal static partial class Helpers
     {
         public const string DefaultSystemModule = "System.Private.CoreLib";
 
-        public static string[] ValidOS { get; } = ["windows", "linux", "freebsd", "osx", "maccatalyst", "ios", "iossimulator", "tvos", "tvossimulator", "android", "browser", "wasi"];
+        public static string[] ValidOS { get; } = ["windows", "linux", "freebsd", "openbsd", "osx", "maccatalyst", "ios", "iossimulator", "tvos", "tvossimulator", "android", "browser", "wasi"];
         public static string[] ValidArchitectures { get; } = ["arm", "armel", "arm64", "x86", "x64", "riscv64", "loongarch64", "wasm"];
 
         public static Dictionary<string, string> BuildPathDictionary(IReadOnlyList<Token> tokens, bool strict)
@@ -70,7 +70,8 @@ public static TargetOS GetTargetOS(string token)
                     return TargetOS.OSX;
                 else if (RuntimeInformation.IsOSPlatform(OSPlatform.FreeBSD))
                     return TargetOS.FreeBSD;
-
+                else if (RuntimeInformation.IsOSPlatform(OSPlatform.Create("OPENBSD")))
+                    return TargetOS.OpenBSD;
                 throw new NotImplementedException();
             }
 
@@ -80,6 +81,7 @@ public static TargetOS GetTargetOS(string token)
                 "win" or "windows" => TargetOS.Windows,
                 "osx" => TargetOS.OSX,
                 "freebsd" => TargetOS.FreeBSD,
+                "openbsd" => TargetOS.OpenBSD,
                 "maccatalyst" => TargetOS.MacCatalyst,
                 "iossimulator" => TargetOS.iOSSimulator,
                 "ios" => TargetOS.iOS,
diff --git a/src/coreclr/tools/Common/Compiler/ObjectWriter/PETargetExtensions.cs b/src/coreclr/tools/Common/Compiler/ObjectWriter/PETargetExtensions.cs
index 12f992c29677b8..1a76c9c6b4ac5f 100644
--- a/src/coreclr/tools/Common/Compiler/ObjectWriter/PETargetExtensions.cs
+++ b/src/coreclr/tools/Common/Compiler/ObjectWriter/PETargetExtensions.cs
@@ -19,6 +19,7 @@ internal enum MachineOSOverride : ushort
         Apple = 0x4644,
         FreeBSD = 0xADC4,
         NetBSD = 0x1993,
+        OpenBSD = 0xADC5,
         SunOS = 0x1992,
     }
 
@@ -125,6 +126,9 @@ public static MachineOSOverride MachineOSOverrideFromTarget(this TargetDetails t
                 case TargetOS.NetBSD:
                     return MachineOSOverride.NetBSD;
 
+                case TargetOS.OpenBSD:
+                    return MachineOSOverride.OpenBSD;
+
                 default:
                     throw new NotImplementedException(target.OperatingSystem.ToString());
             }
diff --git a/src/coreclr/tools/Common/TypeSystem/Common/TargetDetails.cs b/src/coreclr/tools/Common/TypeSystem/Common/TargetDetails.cs
index fc69a28bdcb977..4c9862cd0c8698 100644
--- a/src/coreclr/tools/Common/TypeSystem/Common/TargetDetails.cs
+++ b/src/coreclr/tools/Common/TypeSystem/Common/TargetDetails.cs
@@ -22,6 +22,7 @@ public enum TargetOS
         tvOSSimulator,
         FreeBSD,
         NetBSD,
+        OpenBSD,
         SunOS,
         Browser,
         Wasi
diff --git a/src/coreclr/tools/aot/ILCompiler.Diagnostics/PerfMapWriter.cs b/src/coreclr/tools/aot/ILCompiler.Diagnostics/PerfMapWriter.cs
index 16f82023ad0b57..a5774d7f37b8a6 100644
--- a/src/coreclr/tools/aot/ILCompiler.Diagnostics/PerfMapWriter.cs
+++ b/src/coreclr/tools/aot/ILCompiler.Diagnostics/PerfMapWriter.cs
@@ -116,6 +116,7 @@ private static PerfmapTokensForTarget TranslateTargetDetailsToPerfmapConstants(T
                 TargetOS.OSX => PerfMapOSToken.OSX,
                 TargetOS.FreeBSD => PerfMapOSToken.FreeBSD,
                 TargetOS.NetBSD => PerfMapOSToken.NetBSD,
+                TargetOS.OpenBSD => PerfMapOSToken.OpenBSD,
                 TargetOS.SunOS => PerfMapOSToken.SunOS,
                 _ => throw new NotImplementedException(details.OperatingSystem.ToString())
             };
diff --git a/src/coreclr/tools/aot/ILCompiler.Diagnostics/ReadyToRunDiagnosticsConstants.cs b/src/coreclr/tools/aot/ILCompiler.Diagnostics/ReadyToRunDiagnosticsConstants.cs
index 281349be80e52e..449b80add0a458 100644
--- a/src/coreclr/tools/aot/ILCompiler.Diagnostics/ReadyToRunDiagnosticsConstants.cs
+++ b/src/coreclr/tools/aot/ILCompiler.Diagnostics/ReadyToRunDiagnosticsConstants.cs
@@ -32,6 +32,7 @@ public enum PerfMapOSToken : uint
     FreeBSD = 4,
     NetBSD = 5,
     SunOS = 6,
+    OpenBSD = 7,
 }
 
 public enum PerfMapAbiToken : uint
diff --git a/src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/TargetExtensions.cs b/src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/TargetExtensions.cs
index ef76a9abcce5bb..d6d5189ef6d1c4 100644
--- a/src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/TargetExtensions.cs
+++ b/src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/TargetExtensions.cs
@@ -19,6 +19,7 @@ public enum MachineOSOverride : ushort
         Apple = 0x4644,
         FreeBSD = 0xADC4,
         NetBSD = 0x1993,
+        OpenBSD = 0xADC5,
         SunOS = 0x1992,
     }
 
@@ -125,6 +126,9 @@ public static MachineOSOverride MachineOSOverrideFromTarget(this TargetDetails t
                 case TargetOS.NetBSD:
                     return MachineOSOverride.NetBSD;
 
+                case TargetOS.OpenBSD:
+                    return MachineOSOverride.OpenBSD;
+
                 default:
                     throw new NotImplementedException(target.OperatingSystem.ToString());
             }
diff --git a/src/coreclr/tools/aot/ILCompiler.Reflection.ReadyToRun/ReadyToRunReader.cs b/src/coreclr/tools/aot/ILCompiler.Reflection.ReadyToRun/ReadyToRunReader.cs
index 3fe30ed575c6f8..9ade1797a4088f 100644
--- a/src/coreclr/tools/aot/ILCompiler.Reflection.ReadyToRun/ReadyToRunReader.cs
+++ b/src/coreclr/tools/aot/ILCompiler.Reflection.ReadyToRun/ReadyToRunReader.cs
@@ -28,6 +28,7 @@ public enum OperatingSystem
         FreeBSD = 0xADC4,
         Linux = 0x7B79,
         NetBSD = 0x1993,
+        OpenBSD = 0xADC5,
         SunOS = 0x1992,
         Windows = 0,
         Unknown = -1
diff --git a/src/coreclr/tools/r2rdump/Program.cs b/src/coreclr/tools/r2rdump/Program.cs
index 73539ee312e886..513f60f4df7854 100644
--- a/src/coreclr/tools/r2rdump/Program.cs
+++ b/src/coreclr/tools/r2rdump/Program.cs
@@ -241,7 +241,9 @@ public void Dump(ReadyToRunReader r2r)
                     OperatingSystem.Linux => TargetOS.Linux,
                     OperatingSystem.Apple => TargetOS.OSX,
                     OperatingSystem.FreeBSD => TargetOS.FreeBSD,
-                    OperatingSystem.NetBSD => TargetOS.FreeBSD,
+                    OperatingSystem.NetBSD => TargetOS.NetBSD,
+                    OperatingSystem.OpenBSD => TargetOS.OpenBSD,
+                    OperatingSystem.SunOS => TargetOS.SunOS,
                     OperatingSystem.Unknown => TargetOS.Unknown, // Webcil/WASM images don't encode OS
                     _ => throw new NotImplementedException(r2r.OperatingSystem.ToString()),
                 };
diff --git a/src/coreclr/tools/superpmi/superpmi-shim-collector/CMakeLists.txt b/src/coreclr/tools/superpmi/superpmi-shim-collector/CMakeLists.txt
index 87383b7dc5cb98..b51f935668ab3d 100644
--- a/src/coreclr/tools/superpmi/superpmi-shim-collector/CMakeLists.txt
+++ b/src/coreclr/tools/superpmi/superpmi-shim-collector/CMakeLists.txt
@@ -46,7 +46,7 @@ else()
 
     generate_exports_file(${SPMI_COLLECTOR_EXPORTS} ${SPMI_COLLECTOR_EXPORTS_FINAL_FILE})
 
-    if(CLR_CMAKE_TARGET_LINUX OR CLR_CMAKE_TARGET_FREEBSD OR CLR_CMAKE_TARGET_NETBSD OR CLR_CMAKE_TARGET_SUNOS)
+    if(CLR_CMAKE_TARGET_LINUX OR CLR_CMAKE_TARGET_FREEBSD OR CLR_CMAKE_TARGET_NETBSD OR CLR_CMAKE_TARGET_OPENBSD OR CLR_CMAKE_TARGET_SUNOS)
         # This is required to force using our own PAL, not one that we are loaded with.
         add_link_options(LINKER:-Bsymbolic)
     endif()
diff --git a/src/coreclr/vm/CMakeLists.txt b/src/coreclr/vm/CMakeLists.txt
index 7d6945c79e37d9..f8f2c34da22009 100644
--- a/src/coreclr/vm/CMakeLists.txt
+++ b/src/coreclr/vm/CMakeLists.txt
@@ -17,9 +17,11 @@ include_directories(${CLR_SRC_NATIVE_DIR}/libs/Common)
 add_definitions(-DUNICODE)
 add_definitions(-D_UNICODE)
 
-if(CLR_CMAKE_TARGET_ANDROID)
+if(CLR_CMAKE_TARGET_ANDROID OR CLR_CMAKE_TARGET_OPENBSD)
+    # OpenBSD's ld.so cannot resolve native TLS relocations in shared objects and has no
+    # __tls_get_addr, so the runtime must use emulated TLS (like Android).
     add_definitions(-DFEATURE_EMULATED_TLS)
-endif(CLR_CMAKE_TARGET_ANDROID)
+endif()
 
 foreach (Config DEBUG CHECKED)
     add_compile_definitions($<$<CONFIG:${Config}>:WRITE_BARRIER_CHECK>)
diff --git a/src/coreclr/vm/amd64/asmhelpers.S b/src/coreclr/vm/amd64/asmhelpers.S
index 2d71f26409bc41..17513f82213afa 100644
--- a/src/coreclr/vm/amd64/asmhelpers.S
+++ b/src/coreclr/vm/amd64/asmhelpers.S
@@ -296,7 +296,7 @@ LEAF_END GetThreadVarsAddress, _TEXT
 #ifndef TARGET_APPLE
 # EXTERN_C void* GetTlsIndexObjectDescOffset();
 
-#ifndef TARGET_ANDROID
+#ifndef FEATURE_EMULATED_TLS
 
 # <NOTE>
 # Helper to calculate the offset of native thread local variable `t_ThreadStatics`. The offset has to be found at runtime
@@ -320,7 +320,7 @@ LEAF_ENTRY GetTlsIndexObjectDescOffset, _TEXT
         call    EXTERNAL_C_FUNC(__tls_get_addr)                      # dummy call to have linker see the code pattern to replace the offset
         int 3
 LEAF_END GetTlsIndexObjectDescOffset, _TEXT
-#endif // !TARGET_ANDROID
+#endif // !FEATURE_EMULATED_TLS
 #endif // !TARGET_OSX
 
 LEAF_ENTRY  JIT_PollGC, _TEXT
diff --git a/src/coreclr/vm/threadstatics.cpp b/src/coreclr/vm/threadstatics.cpp
index e4d94eac463453..9dc21a6f7086b8 100644
--- a/src/coreclr/vm/threadstatics.cpp
+++ b/src/coreclr/vm/threadstatics.cpp
@@ -1031,6 +1031,8 @@ bool CanJITOptimizeTLSAccess()
     // Optimization is disabled for FreeBSD/arm64
 #elif defined(TARGET_ANDROID)
     // Optimation is disabled for Android until emulated TLS is supported.
+#elif defined(TARGET_OPENBSD)
+    // Optimization is disabled for OpenBSD, which has no addressable __tls_get_addr.
 #elif !defined(TARGET_APPLE) && defined(TARGET_UNIX) && (defined(TARGET_ARM64) || defined(TARGET_LOONGARCH64))
     bool tlsResolverValid = IsValidTLSResolver();
     if (tlsResolverValid)
@@ -1172,13 +1174,17 @@ void GetThreadLocalStaticBlocksInfo(CORINFO_THREAD_STATIC_BLOCKS_INFO* pInfo)
 
     pInfo->threadVarsSection = GetThreadVarsSectionAddress();
 
-#elif defined(TARGET_AMD64)
+#elif defined(TARGET_AMD64) && !defined(TARGET_OPENBSD)
 
     // For Linux/x64, get the address of tls_get_addr system method and the base address
     // of struct that we will pass to it.
     pInfo->tlsGetAddrFtnPtr = reinterpret_cast<void*>(&__tls_get_addr);
     pInfo->tlsIndexObject = GetTlsIndexObjectAddress();
 
+#elif defined(TARGET_OPENBSD)
+
+    // Unreachable: TLS optimization is disabled on OpenBSD (no addressable __tls_get_addr).
+
 #elif defined(TARGET_ARM64) || defined(TARGET_LOONGARCH64) || defined(TARGET_RISCV64)
 
     // For Linux arm64/loongarch64/riscv64, just get the offset of thread static variable, and during execution,
diff --git a/src/installer/pkg/projects/netcoreappRIDs.props b/src/installer/pkg/projects/netcoreappRIDs.props
index 6c84841697b420..b1c09f223536c1 100644
--- a/src/installer/pkg/projects/netcoreappRIDs.props
+++ b/src/installer/pkg/projects/netcoreappRIDs.props
@@ -9,11 +9,6 @@
     <OfficialBuildRID Include="osx-arm64">
       <Platform>arm64</Platform>
     </OfficialBuildRID>
-    <!-- Not currently built by CoreFX. -->
-    <!-- <OfficialBuildRID Include="netbsd-x64" /> -->
-    <!-- <OfficialBuildRID Include="illumos-x64" /> -->
-    <!-- <OfficialBuildRID Include="solaris-x64" /> -->
-    <!-- <OfficialBuildRID Include="haiku-x64" /> -->
     <OfficialBuildRID Include="win-x86">
       <Platform>x86</Platform>
     </OfficialBuildRID>
@@ -51,6 +46,11 @@
          in our runtime.json to enable others to provide them.  -->
     <UnofficialBuildRID Include="freebsd-x64" />
     <UnofficialBuildRID Include="freebsd-arm64" />
+    <UnofficialBuildRID Include="netbsd-x64" />
+    <UnofficialBuildRID Include="openbsd-x64" />
+    <UnofficialBuildRID Include="illumos-x64" />
+    <UnofficialBuildRID Include="solaris-x64" />
+    <UnofficialBuildRID Include="haiku-x64" />
     <UnofficialBuildRID Include="tizen.4.0.0-armel">
       <Platform>armel</Platform>
     </UnofficialBuildRID>
diff --git a/src/native/corehost/apphost/static/configure.cmake b/src/native/corehost/apphost/static/configure.cmake
index d479257d438740..6734a6e2f54850 100644
--- a/src/native/corehost/apphost/static/configure.cmake
+++ b/src/native/corehost/apphost/static/configure.cmake
@@ -8,8 +8,8 @@ option(HeimdalGssApi "use heimdal implementation of GssApi" OFF)
 
 if (CLR_CMAKE_TARGET_OPENBSD)
     set(HeimdalGssApi ON)
-    set(CMAKE_REQUIRED_INCLUDES ${CROSS_ROOTFS}/heimdal/include)
-    set(CMAKE_PREFIX_PATH ${CROSS_ROOTFS}/heimdal/lib)
+    set(CMAKE_REQUIRED_INCLUDES ${CROSS_ROOTFS}/usr/local/heimdal/include)
+    set(CMAKE_PREFIX_PATH ${CROSS_ROOTFS}/usr/local/heimdal/lib)
 endif()
 
 if (HeimdalGssApi)
diff --git a/src/native/eventpipe/ds-ipc-pal-socket.c b/src/native/eventpipe/ds-ipc-pal-socket.c
index c668c0851f070d..53bd3270af495c 100644
--- a/src/native/eventpipe/ds-ipc-pal-socket.c
+++ b/src/native/eventpipe/ds-ipc-pal-socket.c
@@ -310,7 +310,7 @@ inline
 ds_ipc_mode_t
 ipc_socket_set_default_umask (void)
 {
-#if defined(DS_IPC_PAL_AF_UNIX) && (defined(__APPLE__) || defined(__FreeBSD__))
+#if defined(DS_IPC_PAL_AF_UNIX) && (defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__))
 	// This will set the default permission bit to 600
 	return umask (~(S_IRUSR | S_IWUSR));
 #else
@@ -323,7 +323,7 @@ inline
 void
 ipc_socket_reset_umask (ds_ipc_mode_t mode)
 {
-#if defined(DS_IPC_PAL_AF_UNIX) && (defined(__APPLE__) || defined(__FreeBSD__))
+#if defined(DS_IPC_PAL_AF_UNIX) && (defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__))
 	umask (mode);
 #endif
 }
@@ -431,7 +431,7 @@ inline
 int
 ipc_socket_set_permission (ds_ipc_socket_t s)
 {
-#if defined(DS_IPC_PAL_AF_UNIX) && !(defined(__APPLE__) || defined(__FreeBSD__) || defined(__HAIKU__))
+#if defined(DS_IPC_PAL_AF_UNIX) && !(defined(__APPLE__) || defined(__FreeBSD__) || defined(__HAIKU__) || defined(__OpenBSD__))
 	int result_fchmod;
 	DS_ENTER_BLOCKING_PAL_SECTION;
 	do {
diff --git a/src/native/libs/System.Net.Security.Native/extra_libs.cmake b/src/native/libs/System.Net.Security.Native/extra_libs.cmake
index 23f360d64cd1e1..4fe02e20192e2d 100644
--- a/src/native/libs/System.Net.Security.Native/extra_libs.cmake
+++ b/src/native/libs/System.Net.Security.Native/extra_libs.cmake
@@ -9,8 +9,7 @@ macro(append_extra_security_libs NativeLibsExtra)
      set(_heimdal_hints)
      if (CLR_CMAKE_TARGET_OPENBSD)
         list(APPEND _heimdal_hints
-            "${CMAKE_SYSROOT}/heimdal/lib"
-            "${CMAKE_SYSROOT}/usr/heimdal/lib")
+            "${CMAKE_SYSROOT}/usr/local/heimdal/lib")
      endif()
 
      if (_heimdal_hints)
diff --git a/src/native/libs/System.Security.Cryptography.Native/CMakeLists.txt b/src/native/libs/System.Security.Cryptography.Native/CMakeLists.txt
index 720d6237e9dac7..8c6605503a8e98 100644
--- a/src/native/libs/System.Security.Cryptography.Native/CMakeLists.txt
+++ b/src/native/libs/System.Security.Cryptography.Native/CMakeLists.txt
@@ -19,7 +19,7 @@ add_definitions(-DOPENSSL_API_COMPAT=0x10100000L)
 # is installed under versioned eopenssl* prefixes. In distro-agnostic mode,
 # prefer the newest packaged OpenSSL discovered in the sysroot.
 if (CLR_CMAKE_TARGET_OPENBSD AND FEATURE_DISTRO_AGNOSTIC_SSL)
-    file(GLOB _openbsd_eopenssl_configs "${CMAKE_SYSROOT}/lib/eopenssl*/cmake/OpenSSL/OpenSSLConfig.cmake")
+    file(GLOB _openbsd_eopenssl_configs "${CMAKE_SYSROOT}/usr/local/lib/eopenssl*/cmake/OpenSSL/OpenSSLConfig.cmake")
     if (_openbsd_eopenssl_configs)
         list(SORT _openbsd_eopenssl_configs)
         list(REVERSE _openbsd_eopenssl_configs)
@@ -34,8 +34,8 @@ if (CLR_CMAKE_TARGET_OPENBSD AND FEATURE_DISTRO_AGNOSTIC_SSL)
         unset(OPENSSL_SSL_LIBRARY CACHE)
         set(OpenSSL_DIR "${_openbsd_openssl_dir}" CACHE PATH "OpenSSL config directory" FORCE)
 
-        if (EXISTS "${CMAKE_SYSROOT}/include/${_openbsd_openssl_name}")
-            set(OPENSSL_INCLUDE_DIR "${CMAKE_SYSROOT}/include/${_openbsd_openssl_name}" CACHE PATH "OpenSSL include directory" FORCE)
+        if (EXISTS "${CMAKE_SYSROOT}/usr/local/include/${_openbsd_openssl_name}")
+            set(OPENSSL_INCLUDE_DIR "${CMAKE_SYSROOT}/usr/local/include/${_openbsd_openssl_name}" CACHE PATH "OpenSSL include directory" FORCE)
         endif()
     endif()
 endif()
diff --git a/src/native/libs/configure.cmake b/src/native/libs/configure.cmake
index 22c3837bce100b..4dcb1409f99233 100644
--- a/src/native/libs/configure.cmake
+++ b/src/native/libs/configure.cmake
@@ -22,8 +22,8 @@ elseif (CLR_CMAKE_TARGET_FREEBSD)
     include_directories(SYSTEM ${CROSS_ROOTFS}/usr/local/include)
     set(CMAKE_REQUIRED_INCLUDES ${CROSS_ROOTFS}/usr/local/include)
 elseif (CLR_CMAKE_TARGET_OPENBSD)
-    include_directories(SYSTEM ${CROSS_ROOTFS}/usr/local/include ${CROSS_ROOTFS}/heimdal/include)
-    set(CMAKE_REQUIRED_INCLUDES ${CROSS_ROOTFS}/usr/local/include ${CROSS_ROOTFS}/heimdal/include)
+    include_directories(SYSTEM ${CROSS_ROOTFS}/usr/local/include ${CROSS_ROOTFS}/usr/local/heimdal/include)
+    set(CMAKE_REQUIRED_INCLUDES ${CROSS_ROOTFS}/usr/local/include ${CROSS_ROOTFS}/usr/local/heimdal/include)
 elseif (CLR_CMAKE_TARGET_SUNOS)
     # requires /opt/tools when building in Global Zone (GZ)
     include_directories(SYSTEM /opt/local/include /opt/tools/include)
diff --git a/src/native/minipal/thread.h b/src/native/minipal/thread.h
index 2abcb82e72e4a9..bc1e8c39144501 100644
--- a/src/native/minipal/thread.h
+++ b/src/native/minipal/thread.h
@@ -14,7 +14,10 @@
 #if defined(__linux__)
 #include <unistd.h>
 #include <sys/syscall.h>
-#elif defined(__FreeBSD__) || defined(__OpenBSD__)
+#elif defined(__FreeBSD__)
+#include <pthread_np.h>
+#elif defined(__OpenBSD__)
+#include <unistd.h>
 #include <pthread_np.h>
 #elif defined(__NetBSD__)
 #include <lwp.h>
diff --git a/src/native/minipal/types.h b/src/native/minipal/types.h
index 997e9db21c97d6..3fbc6170468e75 100644
--- a/src/native/minipal/types.h
+++ b/src/native/minipal/types.h
@@ -6,7 +6,7 @@
 
 #include <stdlib.h>
 
-#if defined(TARGET_32BIT) || defined(TARGET_OSX) || defined(TARGET_WINDOWS)
+#if defined(TARGET_32BIT) || defined(TARGET_OSX) || defined(TARGET_WINDOWS) || defined(TARGET_OPENBSD)
 #define FORMAT_PREFIX "l"
 #else
 #define FORMAT_PREFIX ""