diff --git a/clang-tools-extra/clang-tidy/llvmlibc/CMakeLists.txt b/clang-tools-extra/clang-tidy/llvmlibc/CMakeLists.txt
--- a/clang-tools-extra/clang-tidy/llvmlibc/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/llvmlibc/CMakeLists.txt
@@ -1,6 +1,7 @@
 set(LLVM_LINK_COMPONENTS support)
 
 add_clang_library(clangTidyLLVMLibcModule
+  ImplementationInNamespaceCheck.cpp
   LLVMLibcTidyModule.cpp
   RestrictSystemLibcHeadersCheck.cpp
 
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.h b/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.h
@@ -0,0 +1,38 @@
+//===--- ImplementationInNamespaceCheck.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_LLVMLIBC_IMPLEMENTATIONINNAMESPACECHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVMLIBC_IMPLEMENTATIONINNAMESPACECHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang {
+namespace tidy {
+namespace llvm_libc {
+
+/// Checks all llvm-libc implementation is within the correct namespace.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/llvmlibc-implementation-in-namespace.html
+class ImplementationInNamespaceCheck : public ClangTidyCheck {
+public:
+  ImplementationInNamespaceCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace llvm_libc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVMLIBC_IMPLEMENTATIONINNAMESPACECHECK_H
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.cpp b/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.cpp
@@ -0,0 +1,49 @@
+//===--- ImplementationInNamespaceCheck.cpp - clang-tidy ------------------===//
+//
+// 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 "ImplementationInNamespaceCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace llvm_libc {
+
+const static StringRef RequiredNamespace = "__llvm_libc";
+void ImplementationInNamespaceCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      decl(hasParent(translationUnitDecl()), unless(linkageSpecDecl()))
+          .bind("child_of_translation_unit"),
+      this);
+}
+
+void ImplementationInNamespaceCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *MatchedDecl =
+      Result.Nodes.getNodeAs<Decl>("child_of_translation_unit");
+  if (!Result.SourceManager->isInMainFile(MatchedDecl->getLocation()))
+    return;
+
+  if (const auto *NS = dyn_cast<NamespaceDecl>(MatchedDecl)) {
+    if (NS->getName() != RequiredNamespace) {
+      diag(NS->getLocation(), "'%0' needs to be the outermost namespace")
+          << RequiredNamespace;
+    }
+    return;
+  }
+  diag(MatchedDecl->getLocation(),
+       "declaration must be declared within the '%0' namespace")
+      << RequiredNamespace;
+  return;
+}
+
+} // namespace llvm_libc
+} // namespace tidy
+} // namespace clang
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/LLVMLibcTidyModule.cpp b/clang-tools-extra/clang-tidy/llvmlibc/LLVMLibcTidyModule.cpp
--- a/clang-tools-extra/clang-tidy/llvmlibc/LLVMLibcTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/llvmlibc/LLVMLibcTidyModule.cpp
@@ -9,6 +9,7 @@
 #include "../ClangTidy.h"
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
+#include "ImplementationInNamespaceCheck.h"
 #include "RestrictSystemLibcHeadersCheck.h"
 
 namespace clang {
@@ -18,6 +19,8 @@
 class LLVMLibcModule : public ClangTidyModule {
 public:
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
+    CheckFactories.registerCheck<ImplementationInNamespaceCheck>(
+        "llvmlibc-implementation-in-namespace");
     CheckFactories.registerCheck<RestrictSystemLibcHeadersCheck>(
         "llvmlibc-restrict-system-libc-headers");
   }
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -113,6 +113,11 @@
   Flags use of the `C` standard library functions ``memset``, ``memcpy`` and
   ``memcmp`` and similar derivatives on non-trivial types.
 
+- New :doc:`llvmlibc-implementation-in-namespace
+  <clang-tidy/checks/llvmlibc-implementation-in-namespace>` check.
+
+  Checks all llvm-libc implementation is within the correct namespace.
+
 - New :doc:`llvmlibc-restrict-system-libc-headers
   <clang-tidy/checks/llvmlibc-restrict-system-libc-headers>` check.
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -188,6 +188,7 @@
    `llvm-prefer-isa-or-dyn-cast-in-conditionals <llvm-prefer-isa-or-dyn-cast-in-conditionals.html>`_, "Yes"
    `llvm-prefer-register-over-unsigned <llvm-prefer-register-over-unsigned.html>`_, "Yes"
    `llvm-twine-local <llvm-twine-local.html>`_, "Yes"
+   `llvmlibc-implementation-in-namespace <llvmlibc-implementation-in-namespace.html>`_,
    `llvmlibc-restrict-system-libc-headers <llvmlibc-restrict-system-libc-headers.html>`_, "Yes"
    `misc-definitions-in-headers <misc-definitions-in-headers.html>`_, "Yes"
    `misc-misplaced-const <misc-misplaced-const.html>`_,
diff --git a/clang-tools-extra/docs/clang-tidy/checks/llvmlibc-implementation-in-namespace.rst b/clang-tools-extra/docs/clang-tidy/checks/llvmlibc-implementation-in-namespace.rst
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/llvmlibc-implementation-in-namespace.rst
@@ -0,0 +1,28 @@
+.. title:: clang-tidy - llvmlibc-implementation-in-namespace
+
+llvmlibc-implementation-in-namespace
+====================================
+
+Checks that all declarations in the llvm-libc implementation are within the
+correct namespace.
+
+.. code-block:: c++
+
+    // Correct: implementation inside the correct namespace.
+    namespace __llvm_libc {
+        void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}
+        // Namespaces within __llvm_libc namespace are allowed.
+        namespace inner{
+            int localVar = 0;
+        }
+        // Functions with C linkage are allowed.
+        extern "C" void str_fuzz(){}
+    }
+
+    // Incorrect: implementation not in a namespace.
+    void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}
+
+    // Incorrect: outer most namespace is not correct.
+    namespace something_else {
+        void LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {}
+    }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/llvmlibc-implementation-in-namespace.cpp b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc-implementation-in-namespace.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc-implementation-in-namespace.cpp
@@ -0,0 +1,40 @@
+// RUN: %check_clang_tidy %s llvmlibc-implementation-in-namespace %t
+
+#define MACRO_A "defining macros outside namespace is valid"
+
+class ClassB;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration must be declared within the '__llvm_libc' namespace
+struct StructC {};
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: declaration must be declared within the '__llvm_libc' namespace
+char *VarD = MACRO_A;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration must be declared within the '__llvm_libc' namespace
+typedef int typeE;
+// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: declaration must be declared within the '__llvm_libc' namespace
+void funcF() {}
+// CHECK-MESSAGES: :[[@LINE-1]]:6: warning: declaration must be declared within the '__llvm_libc' namespace
+
+namespace namespaceG {
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: '__llvm_libc' needs to be the outermost namespace
+namespace __llvm_libc{
+namespace namespaceH {
+class ClassB;
+} // namespace namespaceH
+struct StructC {};
+} // namespace __llvm_libc
+char *VarD = MACRO_A;
+typedef int typeE;
+void funcF() {}
+} // namespace namespaceG
+
+// Wrapped in correct namespace.
+namespace __llvm_libc {
+// Namespaces within __llvim_libc namespace allowed.
+namespace namespaceI {
+class ClassB;
+} // namespace namespaceI
+struct StructC {};
+char *VarD = MACRO_A;
+typedef int typeE;
+void funcF() {}
+extern "C" void extern_funcJ() {}
+} // namespace __llvm_libc