diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -65,6 +65,7 @@
   ctgmath
   ctime
   ctype.h
+  cuchar
   cwchar
   cwctype
   deque
diff --git a/libcxx/include/cuchar b/libcxx/include/cuchar
new file mode 100644
--- /dev/null
+++ b/libcxx/include/cuchar
@@ -0,0 +1,63 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_CUCHAR
+#define _LIBCPP_CUCHAR
+
+/*
+    cuchar synopsis
+
+Macros:
+
+    __STDC_UTF_16__
+    __STDC_UTF_32__
+
+namespace std {
+
+Types:
+
+  mbstate_t
+  size_t
+
+size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps); // since C++20
+size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);                   // since 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);
+size_t c32rtomb(char* s, char32_t c32, mbstate_t* ps);
+
+} // std
+
+*/
+
+#include <__config>
+
+#if __has_include(<uchar.h>)
+#   include <uchar.h>
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if __has_include(<uchar.h>)
+    using ::mbstate_t;
+    using ::size_t;
+
+    using ::mbrtoc16;
+    using ::c16rtomb;
+    using ::mbrtoc32;
+    using ::c32rtomb;
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif  // _LIBCPP_CUCHAR
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -201,7 +201,10 @@
       header "ctime"
       export *
     }
-    // FIXME: <cuchar> is missing.
+    module cuchar {
+      header "cuchar"
+      export *
+    }
     module cwchar {
       header "cwchar"
       export depr.stdio_h
diff --git a/libcxx/test/libcxx/double_include.sh.cpp b/libcxx/test/libcxx/double_include.sh.cpp
--- a/libcxx/test/libcxx/double_include.sh.cpp
+++ b/libcxx/test/libcxx/double_include.sh.cpp
@@ -80,6 +80,7 @@
 #include <ctgmath>
 #include <ctime>
 #include <ctype.h>
+#include <cuchar>
 #include <cwchar>
 #include <cwctype>
 #include <deque>
diff --git a/libcxx/test/libcxx/min_max_macros.compile.pass.cpp b/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
--- a/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
+++ b/libcxx/test/libcxx/min_max_macros.compile.pass.cpp
@@ -116,6 +116,8 @@
 TEST_MACROS();
 #include <ctype.h>
 TEST_MACROS();
+#include <cuchar>
+TEST_MACROS();
 #include <cwchar>
 TEST_MACROS();
 #include <cwctype>
diff --git a/libcxx/test/libcxx/no_assert_include.compile.pass.cpp b/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
--- a/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
+++ b/libcxx/test/libcxx/no_assert_include.compile.pass.cpp
@@ -73,6 +73,7 @@
 #include <ctgmath>
 #include <ctime>
 #include <ctype.h>
+#include <cuchar>
 #include <cwchar>
 #include <cwctype>
 #include <deque>
diff --git a/libcxx/test/libcxx/strings/c.strings/version_cuchar.pass.cpp b/libcxx/test/libcxx/strings/c.strings/version_cuchar.pass.cpp
--- a/libcxx/test/libcxx/strings/c.strings/version_cuchar.pass.cpp
+++ b/libcxx/test/libcxx/strings/c.strings/version_cuchar.pass.cpp
@@ -5,8 +5,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-//
-// XFAIL: *
 
 // <cuchar>
 
diff --git a/libcxx/test/std/strings/c.strings/cuchar.pass.cpp b/libcxx/test/std/strings/c.strings/cuchar.pass.cpp
--- a/libcxx/test/std/strings/c.strings/cuchar.pass.cpp
+++ b/libcxx/test/std/strings/c.strings/cuchar.pass.cpp
@@ -5,8 +5,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-//
-// XFAIL: libc++
+
+// The macOS SDK doesn't contain <uchar.h> yet, so these tests fail.
+// XFAIL: darwin
 
 // <cuchar>
 
@@ -14,8 +15,20 @@
 
 #include "test_macros.h"
 
-int main(int, char**)
-{
+// __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
+
+int main(int, char**) {
+#if TEST_STD_VER > 17 && !defined(_LIBCPP_VERSION) // TODO: implement this in libc++
+    ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc8((char8_t*)0, (char const*)0, size_t(), (mbstate_t*)0)));
+    ASSERT_SAME_TYPE(size_t, decltype(std::c8rtomb((char*)0, char8_t{}, (mbstate_t*)0)));
+#endif
+
+    ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc16((char16_t*)0, (const char*)0, size_t{}, (mbstate_t*)0)));
+    ASSERT_SAME_TYPE(size_t, decltype(std::c16rtomb((char*)0, char16_t{}, (mbstate_t*)0)));
+
+    ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc32((char32_t*)0, (const char*)0, size_t{}, (mbstate_t*)0)));
+    ASSERT_SAME_TYPE(size_t, decltype(std::c16rtomb((char*)0, char32_t{}, (mbstate_t*)0)));
 
-  return 0;
+    return 0;
 }