diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -665,6 +665,7 @@
   __support/musl/xlocale.h
   __support/newlib/xlocale.h
   __support/openbsd/xlocale.h
+  __support/serenity/xlocale.h
   __support/win32/locale_win32.h
   __support/xlocale/__nop_locale_mgmt.h
   __support/xlocale/__posix_l_fallback.h
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -956,7 +956,8 @@
         defined(__APPLE__) ||                                                                                          \
         defined(__MVS__) ||                                                                                            \
         defined(_AIX) ||                                                                                               \
-        defined(__EMSCRIPTEN__)
+        defined(__EMSCRIPTEN__) ||                                                                                     \
+        defined(__serenity__)
 // clang-format on
 #      define _LIBCPP_HAS_THREAD_API_PTHREAD
 #    elif defined(__Fuchsia__)
@@ -1034,7 +1035,7 @@
 #  endif
 
 #  if defined(__BIONIC__) || defined(__NuttX__) || defined(__Fuchsia__) || defined(__wasi__) ||                        \
-      defined(_LIBCPP_HAS_MUSL_LIBC) || defined(__OpenBSD__)
+      defined(_LIBCPP_HAS_MUSL_LIBC) || defined(__OpenBSD__) || defined(__serenity__)
 #    define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #  endif
 
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -50,6 +50,8 @@
 # include <__support/musl/xlocale.h>
 #elif defined(_LIBCPP_HAS_MUSL_LIBC)
 # include <__support/musl/xlocale.h>
+#elif defined(__serenity__)
+# include <__support/serenity/xlocale.h>
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
diff --git a/libcxx/include/__support/serenity/xlocale.h b/libcxx/include/__support/serenity/xlocale.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__support/serenity/xlocale.h
@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_SUPPORT_SERENITY_XLOCALE_H
+#define _LIBCPP_SUPPORT_SERENITY_XLOCALE_H
+
+#if defined(__serenity__)
+
+#  include <__support/xlocale/__nop_locale_mgmt.h>
+#  include <__support/xlocale/__posix_l_fallback.h>
+#  include <__support/xlocale/__strtonum_fallback.h>
+#  include <clocale>
+#  include <cstdlib>
+#  include <ctype.h>
+#  include <cwctype>
+
+#endif // __serenity__
+
+#endif
diff --git a/libcxx/include/locale b/libcxx/include/locale
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -217,7 +217,7 @@
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#  if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__)
+#  if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__) && !defined(__serenity__)
 #    define _LIBCPP_HAS_CATOPEN 1
 #    include <nl_types.h>
 #  endif
diff --git a/libcxx/src/include/config_elast.h b/libcxx/src/include/config_elast.h
--- a/libcxx/src/include/config_elast.h
+++ b/libcxx/src/include/config_elast.h
@@ -35,6 +35,8 @@
 #define _LIBCPP_ELAST 4095
 #elif defined(__APPLE__)
 // No _LIBCPP_ELAST needed on Apple
+#elif defined(__serenity__)
+// No _LIBCPP_ELAST needed on SerenityOS
 #elif defined(__MVS__)
 #define _LIBCPP_ELAST 1160
 #elif defined(_LIBCPP_MSVCRT_LIKE)