diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 778ff67ce174..128ad047072e 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -391,6 +391,7 @@ def warn_reserved_extern_symbol: Warning<
   "identifier %0 is reserved because %select{"
   "<ERROR>|" // ReservedIdentifierStatus::NotReserved
   "it starts with '_' at global scope|"
+  "it starts with '_' and has C language linkage|"
   "it starts with '__'|"
   "it starts with '_' followed by a capital letter|"
   "it contains '__'}1">,
diff --git a/clang/include/clang/Basic/IdentifierTable.h b/clang/include/clang/Basic/IdentifierTable.h
index 803fec8df30c..19c967efcc42 100644
--- a/clang/include/clang/Basic/IdentifierTable.h
+++ b/clang/include/clang/Basic/IdentifierTable.h
@@ -43,6 +43,7 @@ class SourceLocation;
 enum class ReservedIdentifierStatus {
   NotReserved = 0,
   StartsWithUnderscoreAtGlobalScope,
+  StartsWithUnderscoreAndIsExternC,
   StartsWithDoubleUnderscore,
   StartsWithUnderscoreFollowedByCapitalLetter,
   ContainsDoubleUnderscore,
@@ -60,7 +61,8 @@ inline bool isReservedAtGlobalScope(ReservedIdentifierStatus Status) {
 /// example.
 inline bool isReservedInAllContexts(ReservedIdentifierStatus Status) {
   return Status != ReservedIdentifierStatus::NotReserved &&
-         Status != ReservedIdentifierStatus::StartsWithUnderscoreAtGlobalScope;
+         Status != ReservedIdentifierStatus::StartsWithUnderscoreAtGlobalScope &&
+         Status != ReservedIdentifierStatus::StartsWithUnderscoreAndIsExternC;
 }
 
 /// A simple pair of identifier info and location.
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 57d84f2c3439..acc0839dba75 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1089,12 +1089,28 @@ NamedDecl::isReserved(const LangOptions &LangOpts) const {
 
   ReservedIdentifierStatus Status = II->isReserved(LangOpts);
   if (isReservedAtGlobalScope(Status) && !isReservedInAllContexts(Status)) {
-    // Check if we're at TU level or not.
+    // This name is only reserved at global scope. Check if this declaration
+    // conflicts with a global scope declaration.
     if (isa<ParmVarDecl>(this) || isTemplateParameter())
       return ReservedIdentifierStatus::NotReserved;
+
+    // C++ [dcl.link]/7:
+    //   Two declarations [conflict] if [...] one declares a function or
+    //   variable with C language linkage, and the other declares [...] a
+    //   variable that belongs to the global scope.
+    //
+    // Therefore names that are reserved at global scope are also reserved as
+    // names of variables and functions with C language linkage.
     const DeclContext *DC = getDeclContext()->getRedeclContext();
-    if (!DC->isTranslationUnit())
-      return ReservedIdentifierStatus::NotReserved;
+    if (DC->isTranslationUnit())
+      return Status;
+    if (auto *VD = dyn_cast<VarDecl>(this))
+      if (VD->isExternC())
+        return ReservedIdentifierStatus::StartsWithUnderscoreAndIsExternC;
+    if (auto *FD = dyn_cast<FunctionDecl>(this))
+      if (FD->isExternC())
+        return ReservedIdentifierStatus::StartsWithUnderscoreAndIsExternC;
+    return ReservedIdentifierStatus::NotReserved;
   }
 
   return Status;
diff --git a/clang/test/SemaCXX/reserved-identifier.cpp b/clang/test/SemaCXX/reserved-identifier.cpp
index 56fa3873359b..eaa5fe833033 100644
--- a/clang/test/SemaCXX/reserved-identifier.cpp
+++ b/clang/test/SemaCXX/reserved-identifier.cpp
@@ -105,3 +105,10 @@ struct Any {
 #define _Reserved // expected-warning {{macro name is a reserved identifier}}
 #undef _not_reserved
 #undef _Reserved // expected-warning {{macro name is a reserved identifier}}
+
+namespace N {
+  int _namespace_a;
+  extern "C" int _namespace_b; // expected-warning {{identifier '_namespace_b' is reserved because it starts with '_' and has C language linkage}}
+  void _namespace_c();
+  extern "C" void _namespace_d(); // expected-warning {{identifier '_namespace_d' is reserved because it starts with '_' and has C language linkage}}
+}