diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -170,6 +170,7 @@
 LANGOPT(Modules           , 1, 0, "modules semantics")
 COMPATIBLE_LANGOPT(ModulesTS  , 1, 0, "C++ Modules TS syntax")
 COMPATIBLE_LANGOPT(CPlusPlusModules, 1, 0, "C++ modules syntax")
+COMPATIBLE_LANGOPT(HeaderModules, 1, 0, "Clang header modules")
 BENIGN_ENUM_LANGOPT(CompilingModule, CompilingModuleKind, 3, CMK_None,
                     "compiling a module interface")
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2261,6 +2261,10 @@
   LangOpts<"Modules">, Default<!strconcat(fmodules_ts.KeyPath, "||", fcxx_modules.KeyPath)>,
   PosFlag<SetTrue, [CC1Option], "Enable the 'modules' language feature">,
   NegFlag<SetFalse>, BothFlags<[NoXarchOption, CoreOption]>>;
+defm header_modules : BoolFOption<"header-modules",
+  LangOpts<"HeaderModules">, Default<fmodules.KeyPath>,
+  PosFlag<SetTrue, [], "Enable header modules (Clang extension)">,
+  NegFlag<SetFalse>, BothFlags<[CC1Option, NoDriverOption]>>;
 def fmodule_maps : Flag <["-"], "fmodule-maps">, Flags<[CoreOption]>, Alias<fimplicit_module_maps>;
 def fmodule_name_EQ : Joined<["-"], "fmodule-name=">, Group<f_Group>,
   Flags<[NoXarchOption,CC1Option,CoreOption]>, MetaVarName<"<name>">,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -38,6 +38,7 @@
 #include "clang/Driver/SanitizerArgs.h"
 #include "clang/Driver/Types.h"
 #include "clang/Driver/XRayArgs.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Option/ArgList.h"
@@ -3625,6 +3626,15 @@
     HaveModules = true;
   }
 
+  // Disable header modules in C++20 mode, they are not in the C++ standard.
+  // Unless '-fmodules' was specified explicitly.
+  if (HaveModules) {
+    if (HaveClangModules)
+      CmdArgs.push_back("-fheader-modules");
+    else
+      CmdArgs.push_back("-fno-header-modules");
+  }
+
   // -fmodule-maps enables implicit reading of module map files. By default,
   // this is enabled if we are using Clang's flavor of precompiled modules.
   if (Args.hasFlag(options::OPT_fimplicit_module_maps,
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2159,7 +2159,8 @@
   // Determine whether we should try to import the module for this #include, if
   // there is one. Don't do so if precompiled module support is disabled or we
   // are processing this module textually (because we're building the module).
-  if (Action == Enter && File && SuggestedModule && getLangOpts().Modules &&
+  if (Action == Enter && File && SuggestedModule &&
+      getLangOpts().HeaderModules &&
       !isForModuleBuilding(SuggestedModule.getModule(),
                            getLangOpts().CurrentModule,
                            getLangOpts().ModuleName)) {
diff --git a/clang/test/Driver/cpp20-header-module.cpp b/clang/test/Driver/cpp20-header-module.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/cpp20-header-module.cpp
@@ -0,0 +1,19 @@
+// Check header modules are not enabled with C++20 standard and -fmodules-ts.
+//
+// RUN: %clang -fmodules-ts -std=c++20 -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=NO_HEADER_MODULES
+// RUN: %clang -std=c++20 -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=NO_HEADER_MODULES
+// RUN: %clang -std=c++2a -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=NO_HEADER_MODULES
+// RUN: %clang_cl /std:c++latest /Zs -v %s 2>&1 | FileCheck %s --check-prefix=NO_HEADER_MODULES
+//
+// NO_HEADER_MODULES-NOT: -fheader-modules
+// NO_HEADER_MODULES: -cc1
+// NO_HEADER_MODULES-SAME: -fno-header-modules
+//
+// RUN: %clang -fmodules -fmodules-ts -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=HAS_HEADER_MODULES
+// RUN: %clang -fmodules -fmodules-ts -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=HAS_HEADER_MODULES
+// RUN: %clang -fmodules -std=c++20 -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=HAS_HEADER_MODULES
+// RUN: %clang -fmodules -std=c++2a -fsyntax-only -v %s 2>&1 | FileCheck %s --check-prefix=HAS_HEADER_MODULES
+//
+// HAS_HEADER_MODULES-NOT: -fno-header-modules
+// HAS_HEADER_MODULES: -cc1
+// HAS_HEADER_MODULES: -fheader-modules
diff --git a/clang/test/Modules/Inputs/cxx20-and-header-modules/a.h b/clang/test/Modules/Inputs/cxx20-and-header-modules/a.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/cxx20-and-header-modules/a.h
@@ -0,0 +1,12 @@
+#ifndef A_HEADER
+#define A_HEADER
+
+#if INCLUDING && BUILDING_MODULE
+#error "invalid context"
+#endif
+
+struct X {
+  int foo() { return 10; }
+};
+
+#endif
diff --git a/clang/test/Modules/Inputs/cxx20-and-header-modules/a.map b/clang/test/Modules/Inputs/cxx20-and-header-modules/a.map
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/cxx20-and-header-modules/a.map
@@ -0,0 +1,4 @@
+module A {
+  header "a.h"
+  export *
+}
diff --git a/clang/test/Modules/cxx20-and-header-modules.cpp b/clang/test/Modules/cxx20-and-header-modules.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/cxx20-and-header-modules.cpp
@@ -0,0 +1,14 @@
+// RUN: rm -rf %t
+//
+// Check header modules are disabled by default in C++20 mode.
+// RUN: %clang -std=c++20 -fsyntax-only -fno-implicit-modules -fmodules-cache-path=%t -I%S/Inputs/cxx20-and-header-modules -fmodule-map-file=%S/Inputs/cxx20-and-header-modules/a.map %s
+//
+// Also run in the header modules mode.
+// RUN: %clang -std=c++20 -DBUILDING_MODULE -fmodules -fimplicit-modules -fmodules-cache-path=%t -I%S/Inputs/cxx20-and-header-modules -fmodule-map-file=%S/Inputs/cxx20-and-header-modules/a.map %s
+
+#define INCLUDING 1
+#include "a.h"
+
+int main() {
+  return X().foo();
+}