diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1219,6 +1219,21 @@
 #    define _LIBCPP_PACKED
 #  endif
 
+// c8rtomb() and mbrtoc8() were added in C++20 and C23. Support for these
+// functions is gradually being added to existing C libraries. The conditions
+// below check for known C library versions and conditions under which these
+// functions are declared by the C library.
+#  define _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
+// GNU libc 2.36 and newer declare c8rtomb() and mbrtoc8() in C++ modes if
+// __cpp_char8_t is defined or if C2X extensions are enabled. Unfortunately,
+// determining the latter depends on internal GNU libc details. If the
+// __cpp_char8_t feature test macro is not defined, then a char8_t typedef
+// will be declared as well.
+#  if defined(_LIBCPP_GLIBC_PREREQ) && _LIBCPP_GLIBC_PREREQ(2, 36) && \
+      (defined(__cpp_char8_t) || __GLIBC_USE(ISOC2X))
+#    undef _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
+#  endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP___CONFIG
diff --git a/libcxx/include/cuchar b/libcxx/include/cuchar
--- a/libcxx/include/cuchar
+++ b/libcxx/include/cuchar
@@ -25,6 +25,8 @@
   mbstate_t
   size_t
 
+size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps);   // C++20
+size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);                     // C++20
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps);
 size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps);
 size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps);
@@ -49,6 +51,10 @@
 using ::mbstate_t _LIBCPP_USING_IF_EXISTS;
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 
+#if !defined(_LIBCPP_HAS_NO_C8RTOMB_MBRTOC8)
+using ::mbrtoc8 _LIBCPP_USING_IF_EXISTS;
+using ::c8rtomb _LIBCPP_USING_IF_EXISTS;
+#endif
 using ::mbrtoc16 _LIBCPP_USING_IF_EXISTS;
 using ::c16rtomb _LIBCPP_USING_IF_EXISTS;
 using ::mbrtoc32 _LIBCPP_USING_IF_EXISTS;
diff --git a/libcxx/include/uchar.h b/libcxx/include/uchar.h
--- a/libcxx/include/uchar.h
+++ b/libcxx/include/uchar.h
@@ -11,7 +11,7 @@
 #define _LIBCPP_UCHAR_H
 
 /*
-    uchar.h synopsis // since C++11
+    uchar.h synopsis // since C11
 
 Macros:
 
@@ -22,7 +22,12 @@
 
   mbstate_t
   size_t
+  char8_t   // C23, Builtin type in C++20
+  char16_t  // Builtin type in C++11
+  char32_t  // Builtin type in C++11
 
+size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps);     // C23
+size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);                       // C23
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps);
 size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps);
 size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps);
diff --git a/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp b/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
--- a/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
+++ b/libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
@@ -23,6 +23,11 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(size_t, decltype(mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(size_t, decltype(mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 
diff --git a/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp b/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
--- a/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
+++ b/libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
@@ -20,11 +20,14 @@
 
 #include "test_macros.h"
 
-// TODO: Implement mbrtoc8 and c8rtomb, and add tests for those
-
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(std::c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(std::c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 
diff --git a/libcxx/test/std/strings/c.strings/no_c8rtomb_mbrtoc8.verify.cpp b/libcxx/test/std/strings/c.strings/no_c8rtomb_mbrtoc8.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/strings/c.strings/no_c8rtomb_mbrtoc8.verify.cpp
@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+#include <uchar.h>
+
+using U = decltype(::c8rtomb);
+using V = decltype(::mbrtoc8);
+#if defined(_LIBCPP_HAS_NO_C8RTOMB_MBRTOC8)
+// expected-error@-3 {{no member named 'c8rtomb' in the global namespace}}
+// expected-error@-3 {{no member named 'mbrtoc8' in the global namespace}}
+#else
+// expected-no-diagnostics
+#endif
diff --git a/libcxx/test/support/test_macros.h b/libcxx/test/support/test_macros.h
--- a/libcxx/test/support/test_macros.h
+++ b/libcxx/test/support/test_macros.h
@@ -383,6 +383,10 @@
 #  define TEST_HAS_NO_FGETPOS_FSETPOS
 #endif
 
+#if defined(_LIBCPP_HAS_NO_C8RTOMB_MBRTOC8)
+#  define TEST_HAS_NO_C8RTOMB_MBRTOC8
+#endif
+
 #if defined(TEST_COMPILER_CLANG)
 #  define TEST_DIAGNOSTIC_PUSH _Pragma("clang diagnostic push")
 #  define TEST_DIAGNOSTIC_POP _Pragma("clang diagnostic pop")