diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -10,6 +10,35 @@
 #ifndef _LIBCPP_CONFIG
 #define _LIBCPP_CONFIG
 
+// NuttX is sometimes built as a native target.
+// In that case, the __NuttX__ macro is predefined by the compiler.
+// https://github.com/NuttX/buildroot
+//
+// In other cases, __NuttX__ is an ordinary user-definded macro.
+// It's especially the case for NuttX sim, which is a target to run
+// the entire NuttX as a program on the host OS, which can be Linux,
+// macOS, Windows, etc.
+// https://cwiki.apache.org/confluence/display/NUTTX/NuttX+Simulation
+// In that case, the host OS compiler is used to build NuttX.
+// Thus, eg. NuttX sim on macOS is built with __APPLE__.
+// We #undef predefined macros for those possible host OSes here
+// because the OS APIs this library should use are of NuttX,
+// not the host OS.
+#if defined(__NuttX__)
+#undef __linux__
+#undef __APPLE__
+#undef __FreeBSD__
+#undef __GLIBC__
+#undef __NetBSD__
+#undef _WIN32
+#undef __sun__
+#undef _AIX
+// For the current use of the __unix__ macro in this library,
+// NuttX is not __unix__.
+// This might need to be revisited in future.
+#undef __unix__
+#endif
+
 #if defined(_MSC_VER) && !defined(__clang__)
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #    define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
@@ -1125,6 +1154,7 @@
 #  if defined(__FreeBSD__) || \
       defined(__wasi__) || \
       defined(__NetBSD__) || \
+      defined(__NuttX__) || \
       defined(__linux__) || \
       defined(__GNU__) || \
       defined(__APPLE__) || \
@@ -1227,7 +1257,7 @@
 #  endif
 #endif
 
-#if defined(__BIONIC__) || defined(__CloudABI__) ||                            \
+#if defined(__BIONIC__) || defined(__CloudABI__) || defined(__NuttX__) ||      \
     defined(__Fuchsia__) || defined(__wasi__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #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
@@ -21,6 +21,8 @@
 #if defined(_LIBCPP_MSVCRT_LIKE)
 # include <cstring>
 # include <support/win32/locale_win32.h>
+#elif defined(__NuttX__)
+# include <support/nuttx/xlocale.h>
 #elif defined(_AIX)
 # include <support/ibm/xlocale.h>
 #elif defined(__ANDROID__)
diff --git a/libcxx/include/support/nuttx/xlocale.h b/libcxx/include/support/nuttx/xlocale.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/support/nuttx/xlocale.h
@@ -0,0 +1,18 @@
+// -*- C++ -*-
+//===-------------------- support/nuttx/xlocale.h -------------------------===//
+//
+// 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_NUTTX_XLOCALE_H
+#define _LIBCPP_SUPPORT_NUTTX_XLOCALE_H
+
+#if defined(__NuttX__)
+#include <support/xlocale/__posix_l_fallback.h>
+#include <support/xlocale/__strtonum_fallback.h>
+#endif // __NuttX__
+
+#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
@@ -17,10 +17,14 @@
 #include <errno.h>
 #endif
 
+// Note: _LIBCPP_ELAST needs to be defined only on platforms
+// where strerror/strerror_r can't handle out-of-range errno values.
 #if defined(ELAST)
 #define _LIBCPP_ELAST ELAST
 #elif defined(_NEWLIB_VERSION)
 #define _LIBCPP_ELAST __ELASTERROR
+#elif defined(__NuttX__)
+// No _LIBCPP_ELAST needed on NuttX
 #elif defined(__Fuchsia__)
 // No _LIBCPP_ELAST needed on Fuchsia
 #elif defined(__wasi__)
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -30,7 +30,7 @@
 #include "__sso_allocator"
 #if defined(_LIBCPP_MSVCRT) || defined(__MINGW32__)
 #include "support/win32/locale_win32.h"
-#elif !defined(__BIONIC__)
+#elif !defined(__BIONIC__) && !defined(__NuttX__)
 #include <langinfo.h>
 #endif
 #include <stdlib.h>