diff --git a/libc/src/__support/wchar/CMakeLists.txt b/libc/src/__support/wchar/CMakeLists.txt
index 86a47319f278a..db57d6488bd37 100644
--- a/libc/src/__support/wchar/CMakeLists.txt
+++ b/libc/src/__support/wchar/CMakeLists.txt
@@ -21,11 +21,11 @@ add_object_library(
 )
 
 add_object_library(
-  wcrtomb
+  wcrtomb_bounded
   HDRS
-    wcrtomb.h
+    wcrtomb_bounded.h
   SRCS 
-    wcrtomb.cpp
+    wcrtomb_bounded.cpp
   DEPENDS
     libc.hdr.errno_macros
     libc.hdr.types.char32_t
diff --git a/libc/src/__support/wchar/wcrtomb.cpp b/libc/src/__support/wchar/wcrtomb_bounded.cpp
similarity index 67%
rename from libc/src/__support/wchar/wcrtomb.cpp
rename to libc/src/__support/wchar/wcrtomb_bounded.cpp
index a74a6f3ec34a6..3355c08a938ef 100644
--- a/libc/src/__support/wchar/wcrtomb.cpp
+++ b/libc/src/__support/wchar/wcrtomb_bounded.cpp
@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/__support/wchar/wcrtomb.h"
+#include "src/__support/wchar/wcrtomb_bounded.h"
 #include "src/__support/error_or.h"
 #include "src/__support/wchar/character_converter.h"
 #include "src/__support/wchar/mbstate.h"
@@ -21,8 +21,8 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
-ErrorOr<size_t> wcrtomb(char *__restrict s, wchar_t wc,
-                        mbstate *__restrict ps) {
+ErrorOr<size_t> wcrtomb_bounded(char *__restrict s, wchar_t wc,
+                                mbstate *__restrict ps, size_t max_written) {
   static_assert(sizeof(wchar_t) == 4);
 
   CharacterConverter cr(ps);
@@ -30,15 +30,19 @@ ErrorOr<size_t> wcrtomb(char *__restrict s, wchar_t wc,
   if (!cr.isValidState())
     return Error(EINVAL);
 
+  char buf[sizeof(wchar_t) / sizeof(char)];
   if (s == nullptr)
-    return Error(EILSEQ);
+    s = buf;
 
-  int status = cr.push(static_cast<char32_t>(wc));
-  if (status != 0)
-    return Error(EILSEQ);
+  // if cr isnt empty, it should be represented in mbstate already
+  if (cr.isEmpty()) {
+    int status = cr.push(static_cast<char32_t>(wc));
+    if (status != 0)
+      return Error(EILSEQ);
+  }
 
   size_t count = 0;
-  while (!cr.isEmpty()) {
+  while (!cr.isEmpty() && count < max_written) {
     auto utf8 = cr.pop_utf8(); // can never fail as long as the push succeeded
     LIBC_ASSERT(utf8.has_value());
 
@@ -46,6 +50,10 @@ ErrorOr<size_t> wcrtomb(char *__restrict s, wchar_t wc,
     s++;
     count++;
   }
+
+  if (!cr.isEmpty()) // didn't complete the conversion
+    return -1;
+
   return count;
 }
 
diff --git a/libc/src/__support/wchar/wcrtomb.h b/libc/src/__support/wchar/wcrtomb_bounded.h
similarity index 68%
rename from libc/src/__support/wchar/wcrtomb.h
rename to libc/src/__support/wchar/wcrtomb_bounded.h
index bcd39a92a3b76..fb310dcef40a8 100644
--- a/libc/src/__support/wchar/wcrtomb.h
+++ b/libc/src/__support/wchar/wcrtomb_bounded.h
@@ -6,8 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_H
-#define LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_H
+#ifndef LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_BOUNDED_H
+#define LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_BOUNDED_H
 
 #include "hdr/types/size_t.h"
 #include "hdr/types/wchar_t.h"
@@ -18,9 +18,10 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
-ErrorOr<size_t> wcrtomb(char *__restrict s, wchar_t wc, mbstate *__restrict ps);
+ErrorOr<size_t> wcrtomb_bounded(char *__restrict s, wchar_t wc,
+                                mbstate *__restrict ps, size_t max_written);
 
 } // namespace internal
 } // namespace LIBC_NAMESPACE_DECL
 
-#endif // LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_H
+#endif // LLVM_LIBC_SRC__SUPPORT_WCHAR_WCRTOMB_BOUNDED_H
diff --git a/libc/src/wchar/CMakeLists.txt b/libc/src/wchar/CMakeLists.txt
index 16664100d42c7..cc610502f5c7d 100644
--- a/libc/src/wchar/CMakeLists.txt
+++ b/libc/src/wchar/CMakeLists.txt
@@ -44,7 +44,7 @@ add_entrypoint_object(
     libc.hdr.types.wchar_t
     libc.hdr.types.mbstate_t
     libc.src.__support.libc_errno
-    libc.src.__support.wchar.wcrtomb
+    libc.src.__support.wchar.wcrtomb_bounded
     libc.src.__support.wchar.mbstate
 )
 
@@ -56,7 +56,7 @@ add_entrypoint_object(
     wctomb.h
   DEPENDS
     libc.hdr.types.wchar_t
-    libc.src.__support.wchar.wcrtomb
+    libc.src.__support.wchar.wcrtomb_bounded
     libc.src.__support.wchar.mbstate
     libc.src.__support.libc_errno
 )
diff --git a/libc/src/wchar/wcrtomb.cpp b/libc/src/wchar/wcrtomb.cpp
index 3e9df0599431e..56d03e9fe5ebf 100644
--- a/libc/src/wchar/wcrtomb.cpp
+++ b/libc/src/wchar/wcrtomb.cpp
@@ -14,7 +14,7 @@
 #include "src/__support/libc_errno.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/wchar/mbstate.h"
-#include "src/__support/wchar/wcrtomb.h"
+#include "src/__support/wchar/wcrtomb_bounded.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
@@ -23,16 +23,14 @@ LLVM_LIBC_FUNCTION(size_t, wcrtomb,
   static internal::mbstate internal_mbstate;
 
   // when s is nullptr, this is equivalent to wcrtomb(buf, L'\0', ps)
-  char buf[sizeof(wchar_t) / sizeof(char)];
-  if (s == nullptr) {
-    s = buf;
+  if (s == nullptr)
     wc = L'\0';
-  }
 
-  auto result = internal::wcrtomb(
+  auto result = internal::wcrtomb_bounded(
       s, wc,
       ps == nullptr ? &internal_mbstate
-                    : reinterpret_cast<internal::mbstate *>(ps));
+                    : reinterpret_cast<internal::mbstate *>(ps),
+      sizeof(wchar_t));
 
   if (!result.has_value()) {
     libc_errno = result.error();
diff --git a/libc/src/wchar/wctomb.cpp b/libc/src/wchar/wctomb.cpp
index 142302e6ae09b..2b2c67f4dc535 100644
--- a/libc/src/wchar/wctomb.cpp
+++ b/libc/src/wchar/wctomb.cpp
@@ -13,7 +13,7 @@
 #include "src/__support/libc_errno.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/wchar/mbstate.h"
-#include "src/__support/wchar/wcrtomb.h"
+#include "src/__support/wchar/wcrtomb_bounded.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
@@ -22,7 +22,8 @@ LLVM_LIBC_FUNCTION(int, wctomb, (char *s, wchar_t wc)) {
   if (s == nullptr)
     return 0;
 
-  auto result = internal::wcrtomb(s, wc, &internal_mbstate);
+  auto result =
+      internal::wcrtomb_bounded(s, wc, &internal_mbstate, sizeof(wchar_t));
 
   if (!result.has_value()) { // invalid wide character
     libc_errno = EILSEQ;
diff --git a/libc/test/src/__support/wchar/CMakeLists.txt b/libc/test/src/__support/wchar/CMakeLists.txt
index 5176bfd4b024b..44bf4c7038d80 100644
--- a/libc/test/src/__support/wchar/CMakeLists.txt
+++ b/libc/test/src/__support/wchar/CMakeLists.txt
@@ -19,3 +19,14 @@ add_libc_test(
   DEPENDS
     libc.src.__support.wchar.character_converter
 )
+
+add_libc_test(
+  wcrtomb_bounded_test
+  SUITE
+    libc-support-tests
+  SRCS
+    wcrtomb_bounded_test.cpp
+  DEPENDS
+    libc.src.__support.wchar.wcrtomb_bounded
+    libc.src.__support.wchar.mbstate
+)
diff --git a/libc/test/src/__support/wchar/wcrtomb_bounded_test.cpp b/libc/test/src/__support/wchar/wcrtomb_bounded_test.cpp
new file mode 100644
index 0000000000000..8ba5cfb119d29
--- /dev/null
+++ b/libc/test/src/__support/wchar/wcrtomb_bounded_test.cpp
@@ -0,0 +1,129 @@
+//===-- Unittests for wcrtomb_bounded -------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/errno_macros.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/wcrtomb_bounded.h"
+#include "test/UnitTest/Test.h"
+
+// The majority of the following tests are the same as
+// tests/src/wchar/wcrtomb_test.cpp
+
+TEST(LlvmLibcWCRToMBBoundedTest, OneByte) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  wchar_t wc = L'U';
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 4);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(1));
+  ASSERT_EQ(mb[0], 'U');
+}
+
+TEST(LlvmLibcWCRToMBBoundedTest, TwoByte) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  // testing utf32: 0xff -> utf8: 0xc3 0xbf
+  wchar_t wc = 0xff;
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 4);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(2));
+  ASSERT_EQ(mb[0], static_cast<char>(0xc3));
+  ASSERT_EQ(mb[1], static_cast<char>(0xbf));
+}
+
+TEST(LlvmLibcWCRToMBBoundedTest, ThreeByte) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  // testing utf32: 0xac15 -> utf8: 0xea 0xb0 0x95
+  wchar_t wc = 0xac15;
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 4);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(3));
+  ASSERT_EQ(mb[0], static_cast<char>(0xea));
+  ASSERT_EQ(mb[1], static_cast<char>(0xb0));
+  ASSERT_EQ(mb[2], static_cast<char>(0x95));
+}
+
+TEST(LlvmLibcWCRToMBTest, FourByte) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  // testing utf32: 0x1f921 -> utf8: 0xf0 0x9f 0xa4 0xa1
+  wchar_t wc = 0x1f921;
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 4);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(4));
+  ASSERT_EQ(mb[0], static_cast<char>(0xf0));
+  ASSERT_EQ(mb[1], static_cast<char>(0x9f));
+  ASSERT_EQ(mb[2], static_cast<char>(0xa4));
+  ASSERT_EQ(mb[3], static_cast<char>(0xa1));
+}
+
+TEST(LlvmLibcWCRToMBBoundedTest, NullString) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  wchar_t wc = L'A';
+
+  // should return the multi byte length of the widechar
+  auto result =
+      LIBC_NAMESPACE::internal::wcrtomb_bounded(nullptr, wc, &state, 4);
+
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(1));
+}
+
+TEST(LlvmLibcWCRToMBBoundedTest, InvalidWchar) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  wchar_t wc = 0x12ffff;
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 4);
+  ASSERT_FALSE(result.has_value());
+  ASSERT_EQ(result.error(), EILSEQ);
+}
+
+TEST(LlvmLibcWCRToMBBoundedTest, InvalidMBState) {
+  LIBC_NAMESPACE::internal::mbstate inv;
+  inv.total_bytes = 6;
+  wchar_t wc = L'A';
+  char mb[4];
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &inv, 4);
+  ASSERT_FALSE(result.has_value());
+  ASSERT_EQ(result.error(), EINVAL);
+}
+
+// wcrtomb_bounded unique tests
+
+TEST(LlvmLibcWCRToMBBoundedTest, ContinueConversion) {
+  LIBC_NAMESPACE::internal::mbstate state;
+  // utf32: 0x1f921 -> utf8: 0xf0 0x9f 0xa4 0xa1
+  wchar_t wc = 0x1f921;
+  char mb[5] = {'\x01', '\x01', '\x01', '\x01', '\x01'};
+  auto result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb, wc, &state, 1);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(),
+            static_cast<size_t>(-1)); // conversion not completed
+  ASSERT_EQ(mb[0], '\xF0');
+  ASSERT_EQ(mb[1], '\x01');
+
+  result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb + 1, wc, &state, 2);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(),
+            static_cast<size_t>(-1)); // conversion not completed
+  ASSERT_EQ(mb[0], '\xF0');
+  ASSERT_EQ(mb[1], '\x9F');
+  ASSERT_EQ(mb[2], '\xA4');
+  ASSERT_EQ(mb[3], '\x01');
+
+  result = LIBC_NAMESPACE::internal::wcrtomb_bounded(mb + 3, wc, &state, 100);
+  ASSERT_TRUE(result.has_value());
+  ASSERT_EQ(result.value(), static_cast<size_t>(1));
+  ASSERT_EQ(mb[0], '\xF0');
+  ASSERT_EQ(mb[1], '\x9F');
+  ASSERT_EQ(mb[2], '\xA4');
+  ASSERT_EQ(mb[3], '\xA1');
+  ASSERT_EQ(mb[4], '\x01');
+}
\ No newline at end of file