diff --git a/llvm/lib/Support/BLAKE3/CMakeLists.txt b/llvm/lib/Support/BLAKE3/CMakeLists.txt
index 99cb78881ec47..eae2b0280e5d3 100644
--- a/llvm/lib/Support/BLAKE3/CMakeLists.txt
+++ b/llvm/lib/Support/BLAKE3/CMakeLists.txt
@@ -85,3 +85,9 @@ endif()
 add_library(LLVMSupportBlake3 OBJECT EXCLUDE_FROM_ALL ${LLVM_BLAKE3_FILES})
 set_target_properties(LLVMSupportBlake3 PROPERTIES FOLDER "LLVM/Libraries")
 llvm_update_compile_flags(LLVMSupportBlake3)
+if(LLVM_BUILD_LLVM_DYLIB OR BUILD_SHARED_LIBS)
+  # Since LLVMSupportBlake3 is not defined using llvm_add_library(), we must
+  # define LLVM_EXPORTS here so its public interface is annotated with
+  # __declspec(dllexport) when building as a DLL on Windows.
+  target_compile_definitions(LLVMSupportBlake3 PRIVATE LLVM_EXPORTS)
+endif()