diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -78,6 +78,11 @@
   exceeds the maximum supported size, as required by the C++ standard.
   Previously the type ``std::length_error`` was used.
 
+- The C++03 emulation type for ``std::nullptr_t`` has been removed in favour of
+  using ``decltype(nullptr)`` in all Standard modes. This is an ABI break for
+  anyone compiling in C++03 mode and who has ``std::nullptr_t`` as part of their
+  ABI.
+
 Build System Changes
 --------------------
 
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -230,7 +230,6 @@
   __memory/uses_allocator.h
   __mutex_base
   __node_handle
-  __nullptr
   __random/uniform_int_distribution.h
   __ranges/access.h
   __ranges/all.h
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -74,10 +74,6 @@
 #  define _LIBCPP_ABI_FIX_UNORDERED_NODE_POINTER_UB
 #  define _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
 #  define _LIBCPP_ABI_FIX_UNORDERED_CONTAINER_SIZE_TYPE
-// Don't use a nullptr_t simulation type in C++03 instead using C++11 nullptr
-// provided under the alternate keyword __nullptr, which changes the mangling
-// of nullptr_t. This option is ABI incompatible with GCC in C++03 mode.
-#  define _LIBCPP_ABI_ALWAYS_USE_CXX11_NULLPTR
 // Define a key function for `bad_function_call` in the library, to centralize
 // its vtable and typeinfo to libc++ rather than having all other libraries
 // using that class define their own copies.
@@ -459,12 +455,8 @@
 #  define _LIBCPP_NORETURN __attribute__ ((noreturn))
 #endif
 
-#if !(__has_feature(cxx_nullptr))
-#  if (__has_extension(cxx_nullptr) || __has_keyword(__nullptr)) && defined(_LIBCPP_ABI_ALWAYS_USE_CXX11_NULLPTR)
-#    define nullptr __nullptr
-#  else
-#    define _LIBCPP_HAS_NO_NULLPTR
-#  endif
+#ifdef _LIBCPP_CXX03_LANG
+# define nullptr __nullptr
 #endif
 
 // Objective-C++ features (opt-in)
diff --git a/libcxx/include/__debug b/libcxx/include/__debug
--- a/libcxx/include/__debug
+++ b/libcxx/include/__debug
@@ -17,10 +17,6 @@
 #pragma GCC system_header
 #endif
 
-#if defined(_LIBCPP_HAS_NO_NULLPTR)
-# include <cstddef>
-#endif
-
 #if _LIBCPP_DEBUG_LEVEL >= 1 || defined(_LIBCPP_BUILDING_LIBRARY)
 #   include <cstdlib>
 #   include <cstdio>
diff --git a/libcxx/include/__memory/auto_ptr.h b/libcxx/include/__memory/auto_ptr.h
--- a/libcxx/include/__memory/auto_ptr.h
+++ b/libcxx/include/__memory/auto_ptr.h
@@ -11,7 +11,6 @@
 #define _LIBCPP___MEMORY_AUTO_PTR_H
 
 #include <__config>
-#include <__nullptr>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/libcxx/include/__nullptr b/libcxx/include/__nullptr
deleted file mode 100644
--- a/libcxx/include/__nullptr
+++ /dev/null
@@ -1,61 +0,0 @@
-// -*- C++ -*-
-//===--------------------------- __nullptr --------------------------------===//
-//
-// 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_NULLPTR
-#define _LIBCPP_NULLPTR
-
-#include <__config>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
-
-#ifdef _LIBCPP_HAS_NO_NULLPTR
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-struct _LIBCPP_TEMPLATE_VIS nullptr_t
-{
-    void* __lx;
-
-    struct __nat {int __for_bool_;};
-
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR nullptr_t() : __lx(0) {}
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : __lx(0) {}
-
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR operator int __nat::*() const {return 0;}
-
-    template <class _Tp>
-        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-        operator _Tp* () const {return 0;}
-
-    template <class _Tp, class _Up>
-        _LIBCPP_INLINE_VISIBILITY
-        operator _Tp _Up::* () const {return 0;}
-
-    friend _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR bool operator==(nullptr_t, nullptr_t) {return true;}
-    friend _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR bool operator!=(nullptr_t, nullptr_t) {return false;}
-};
-
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR nullptr_t __get_nullptr_t() {return nullptr_t(0);}
-
-#define nullptr _VSTD::__get_nullptr_t()
-
-_LIBCPP_END_NAMESPACE_STD
-
-#else  // _LIBCPP_HAS_NO_NULLPTR
-
-namespace std
-{
-    typedef decltype(nullptr) nullptr_t;
-}
-
-#endif // _LIBCPP_HAS_NO_NULLPTR
-
-#endif // _LIBCPP_NULLPTR
diff --git a/libcxx/include/__support/win32/locale_win32.h b/libcxx/include/__support/win32/locale_win32.h
--- a/libcxx/include/__support/win32/locale_win32.h
+++ b/libcxx/include/__support/win32/locale_win32.h
@@ -11,7 +11,7 @@
 #define _LIBCPP_SUPPORT_WIN32_LOCALE_WIN32_H
 
 #include <__config>
-#include <__nullptr>
+#include <cstddef>
 #include <locale.h> // _locale_t
 #include <stdio.h>
 
diff --git a/libcxx/include/cstddef b/libcxx/include/cstddef
--- a/libcxx/include/cstddef
+++ b/libcxx/include/cstddef
@@ -34,18 +34,16 @@
 */
 
 #include <__config>
+#include <stddef.h>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
-// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
-#include_next <stddef.h>
-#include <__nullptr>
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+using ::nullptr_t;
 using ::ptrdiff_t _LIBCPP_USING_IF_EXISTS;
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -872,7 +872,6 @@
   module __mbstate           { private header "__mbstate_t.h"       export * }
   module __mutex_base        { private header "__mutex_base"        export * }
   module __node_handle       { private header "__node_handle"       export * }
-  module __nullptr           { header "__nullptr"           export * }
   module __split_buffer      { private header "__split_buffer"      export * }
   module __std_stream        { private header "__std_stream"        export * }
   module __string            { private header "__string"            export * }
diff --git a/libcxx/include/stddef.h b/libcxx/include/stddef.h
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -45,12 +45,7 @@
 #include_next <stddef.h>
 
 #ifdef __cplusplus
-
-extern "C++" {
-#include <__nullptr>
-using std::nullptr_t;
-}
-
+    typedef decltype(nullptr) nullptr_t;
 #endif
 
 #endif // _LIBCPP_STDDEF_H
diff --git a/libcxx/test/std/language.support/support.types/nullptr_t.pass.cpp b/libcxx/test/std/language.support/support.types/nullptr_t.pass.cpp
--- a/libcxx/test/std/language.support/support.types/nullptr_t.pass.cpp
+++ b/libcxx/test/std/language.support/support.types/nullptr_t.pass.cpp
@@ -80,14 +80,9 @@
         test_conversions<int A::*>();
     }
     {
-#ifdef _LIBCPP_HAS_NO_NULLPTR
+        // TODO: Enable this assertion when GCC compilers implements http://wg21.link/CWG583.
+#if !defined(TEST_COMPILER_GCC)
         static_assert(!has_less<std::nullptr_t>::value, "");
-        // FIXME: our C++03 nullptr emulation still allows for comparisons
-        // with other pointer types by way of the conversion operator.
-        //static_assert(!has_less<void*>::value, "");
-#else
-        // TODO Enable this assertion when all compilers implement core DR 583.
-        // static_assert(!has_less<std::nullptr_t>::value, "");
 #endif
         test_comparisons<std::nullptr_t>();
         test_comparisons<void*>();
diff --git a/libcxx/utils/generate_private_header_tests.py b/libcxx/utils/generate_private_header_tests.py
--- a/libcxx/utils/generate_private_header_tests.py
+++ b/libcxx/utils/generate_private_header_tests.py
@@ -54,8 +54,7 @@
     return not rp.startswith('__support') and rp not in [
         "__bsd_locale_defaults.h", "__bsd_locale_fallbacks.h", "__config",
         "__config_site.in", "__debug", "__hash_table", "__functional_base",
-        "__libcpp_version", "__nullptr", "__threading_support", "__tree",
-        "__undef_macros"
+        "__libcpp_version", "__threading_support", "__tree", "__undef_macros"
     ]