Index: clang-tidy/readability/CMakeLists.txt
===================================================================
--- clang-tidy/readability/CMakeLists.txt
+++ clang-tidy/readability/CMakeLists.txt
@@ -22,6 +22,7 @@
   ReadabilityTidyModule.cpp
   RedundantControlFlowCheck.cpp
   RedundantDeclarationCheck.cpp
+  RedundantExternCheck.cpp
   RedundantFunctionPtrDereferenceCheck.cpp
   RedundantMemberInitCheck.cpp
   RedundantPreprocessorCheck.cpp
Index: clang-tidy/readability/ReadabilityTidyModule.cpp
===================================================================
--- clang-tidy/readability/ReadabilityTidyModule.cpp
+++ clang-tidy/readability/ReadabilityTidyModule.cpp
@@ -29,6 +29,7 @@
 #include "RedundantControlFlowCheck.h"
 #include "RedundantDeclarationCheck.h"
 #include "RedundantFunctionPtrDereferenceCheck.h"
+#include "RedundantExternCheck.h"
 #include "RedundantMemberInitCheck.h"
 #include "RedundantPreprocessorCheck.h"
 #include "RedundantSmartptrGetCheck.h"
@@ -79,6 +80,8 @@
         "readability-misleading-indentation");
     CheckFactories.registerCheck<MisplacedArrayIndexCheck>(
         "readability-misplaced-array-index");
+    CheckFactories.registerCheck<RedundantExternCheck>(
+        "readability-redundant-extern");
     CheckFactories.registerCheck<RedundantFunctionPtrDereferenceCheck>(
         "readability-redundant-function-ptr-dereference");
     CheckFactories.registerCheck<RedundantMemberInitCheck>(
Index: clang-tidy/readability/RedundantExternCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/readability/RedundantExternCheck.h
@@ -0,0 +1,34 @@
+//===--- RedundantExternCheck.h - clang-tidy --------------------*- C++ -*-===//
+//
+// 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 LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_REDUNDANT_EXTERN_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_REDUNDANT_EXTERN_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace readability {
+
+/// Finds redundant 'extern' keywords
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-extern.html
+class RedundantExternCheck : public ClangTidyCheck {
+public:
+  RedundantExternCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_REDUNDANT_EXTERN_H
Index: clang-tidy/readability/RedundantExternCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/readability/RedundantExternCheck.cpp
@@ -0,0 +1,70 @@
+//===--- RedundantExternCheck.cpp - clang-tidy ------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "RedundantExternCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Lex/Lexer.h"
+
+AST_MATCHER(clang::FunctionDecl, hasExternStorageClass) {
+  return Node.getStorageClass() == clang::SC_Extern;
+}
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace readability {
+
+void RedundantExternCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(functionDecl(hasExternStorageClass(), unless(isExternC()))
+                         .bind("redundant_extern"),
+                     this);
+}
+
+void RedundantExternCheck::check(const MatchFinder::MatchResult &Result) {
+  auto *FD = Result.Nodes.getNodeAs<FunctionDecl>("redundant_extern");
+  SourceLocation BeginLoc = FD->getBeginLoc();
+  SourceLocation EndLoc = FD->getEndLoc();
+
+  StringRef Message;
+
+  if (FD->getLinkageInternal() == InternalLinkage) {
+    Message = "'extern' keyword has no effect";
+  } else {
+    Message = "redundant 'extern' keyword";
+  }
+
+  auto Diag = diag(FD->getBeginLoc(), Message);
+
+  if (FD->getBeginLoc().isMacroID())
+    return;
+
+  StringRef Text =
+      Lexer::getSourceText(CharSourceRange::getTokenRange(BeginLoc, EndLoc),
+                           *Result.SourceManager, getLangOpts());
+
+  llvm::StringLiteral Extern = llvm::StringLiteral("extern");
+  int Offset = Text.find(Extern);
+
+  SourceRange ExternRange(BeginLoc.getLocWithOffset(Offset),
+                          BeginLoc.getLocWithOffset(Offset + Extern.size()));
+
+  StringRef ExternText =
+      Lexer::getSourceText(CharSourceRange::getTokenRange(ExternRange),
+                           *Result.SourceManager, getLangOpts());
+
+  if (!ExternText.equals(Extern)) // Final check: typedefs, etc.
+    return;
+
+  Diag << FixItHint::CreateRemoval(CharSourceRange::getTokenRange(ExternRange));
+}
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -240,6 +240,11 @@
   but either don't specify it or the clause is specified but with the kind
   other than ``none``, and suggests to use the ``default(none)`` clause.
 
+- New :doc:`readability-redundant-extern
+  <clang-tidy/checks/readability-redundant-extern>` check.
+
+  Removes the redundant ``extern`` keywords.
+
 Improvements to clang-include-fixer
 -----------------------------------
 
Index: docs/clang-tidy/checks/readability-redundant-extern.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/readability-redundant-extern.rst
@@ -0,0 +1,30 @@
+.. title:: clang-tidy - readability-redundant-extern
+
+readability-redundant-extern
+============================
+
+Removes the redundant or unnecessary ``extern`` keywords from code.
+
+``extern`` is redundant in function declarations, and has no effect in
+namespaces
+
+The default language linkage is C++, so without any additional parameters
+it is redundant (extern "C++" can also be redundant, but it depends on
+the context). In C context (extern "C") the situation is the same, extern
+keyword is redundant for function declarations
+
+.. code-block:: c++
+
+  extern void h();
+
+  namespace {
+    extern void namespace_1();  
+  }
+
+  #define FOO_EXTERN extern
+  FOO_EXTERN void foo_macro_1();
+
+  #define FOO_EXTERN extern
+  typedef int extern_int;
+  extern_int FOO_EXTERN typedef_extern_foo();
+
Index: test/clang-tidy/readability-redundant-extern.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/readability-redundant-extern.cpp
@@ -0,0 +1,74 @@
+// RUN: %check_clang_tidy %s readability-redundant-extern %t
+
+extern int f();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: int f();
+
+int extern f() { return 0; }
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: int f() { return 0; }
+
+extern "C" int g();
+// CHECK-FIXES: extern "C" int g();
+
+extern "C" int g() { return 0; }
+// CHECK-FIXES: extern "C" int g() { return 0; }
+
+extern "C++" int h();
+// CHECK-FIXES: extern "C++" int h();
+
+extern "C++" int h() { return 0; }
+// CHECK-FIXES: extern "C++" int h() { return 0; }
+
+inline extern void foo_inline();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: inline void foo_inline();
+
+#define FOO_EXTERN extern
+FOO_EXTERN void foo_macro_1();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: FOO_EXTERN void foo_macro_1();
+
+#define FOO_INLINE inline
+FOO_INLINE extern void foo_macro_2();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: FOO_INLINE extern void foo_macro_2();
+
+#define FOO_EXTERN_INLINE inline extern
+FOO_EXTERN_INLINE void foo_macro_3();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: FOO_EXTERN_INLINE void foo_macro_3();
+
+void file_scope();
+// CHECK-FIXES: void file_scope();
+
+void another_file_scope(int _extern);
+// CHECK-FIXES: void another_file_scope(int _extern);
+
+namespace {
+extern void namespace_1();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: 'extern' keyword has no effect [readability-redundant-extern]
+// CHECK-FIXES: void namespace_1();
+}
+
+namespace a {
+namespace {
+namespace b {
+extern void namespace_2();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: 'extern' keyword has no effect [readability-redundant-extern]
+// CHECK-FIXES: void namespace_2();
+}
+}
+}
+
+namespace {
+extern "C" void namespace_3();
+// CHECK-FIXES: extern "C" void namespace_3();
+}
+
+#define FOO_EXTERN extern
+typedef int extern_int;
+
+extern_int FOO_EXTERN typedef_extern_foo();
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: redundant 'extern' keyword [readability-redundant-extern]
+// CHECK-FIXES: extern_int FOO_EXTERN typedef_extern_foo();