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
@@ -6,6 +6,7 @@
 add_clang_library(clangTidyLLVMLibcModule
   CalleeNamespaceCheck.cpp
   ImplementationInNamespaceCheck.cpp
+  InlineFunctionDeclCheck.cpp
   LLVMLibcTidyModule.cpp
   RestrictSystemLibcHeadersCheck.cpp
 
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.h b/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.h
@@ -0,0 +1,38 @@
+//===-- InlineFunctionDeclCheck.h -------------------------------*- 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_INLINEFUNCTIONDECLCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVMLIBC_INLINEFUNCTIONDECLCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::llvm_libc {
+
+/// Checks that explicitly and implicitly inline functions in headers files
+/// are tagged with the LIBC_INLINE macro.
+///
+/// For more information about the LIBC_INLINE macro, see
+/// https://libc.llvm.org/code_style.html.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/llvmlibc/inline-function-decl-check.html
+class InlineFunctionDeclCheck : public ClangTidyCheck {
+public:
+  InlineFunctionDeclCheck(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 clang::tidy::llvm_libc
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_LLVMLIBC_INLINEFUNCTIONDECLCHECK_H
diff --git a/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.cpp b/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/llvmlibc/InlineFunctionDeclCheck.cpp
@@ -0,0 +1,43 @@
+//===-- InlineFunctionDeclCheck.cpp ---------------------------------------===//
+//
+// 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 "InlineFunctionDeclCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+#include "llvm/ADT/StringSet.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::llvm_libc {
+
+void InlineFunctionDeclCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(decl(functionDecl()).bind("func_decl"), this);
+}
+
+void InlineFunctionDeclCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *FuncDecl = Result.Nodes.getNodeAs<FunctionDecl>("func_decl");
+  // Ignore functions which are not inline.
+  if (!FuncDecl->isInlined())
+    return;
+
+  SourceLocation SrcBegin = FuncDecl->getSourceRange().getBegin();
+  // Consider functions only in header files.
+  if (!Result.SourceManager->getFilename(SrcBegin).ends_with(".h"))
+    return;
+  // Check if decl starts with LIBC_INLINE
+  auto Loc = FullSourceLoc(SrcBegin, *Result.SourceManager);
+  if (Loc.getBufferData().starts_with("LIBC_INLINE"))
+    return;
+
+  diag(SrcBegin, "%0 must be tagged with the LIBC_INLINE macro; the macro "
+                 "should be placed at the beginning of the declaration")
+      << FuncDecl;
+}
+
+} // namespace clang::tidy::llvm_libc
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
@@ -11,6 +11,7 @@
 #include "../ClangTidyModuleRegistry.h"
 #include "CalleeNamespaceCheck.h"
 #include "ImplementationInNamespaceCheck.h"
+#include "InlineFunctionDeclCheck.h"
 #include "RestrictSystemLibcHeadersCheck.h"
 
 namespace clang::tidy {
@@ -23,6 +24,8 @@
         "llvmlibc-callee-namespace");
     CheckFactories.registerCheck<ImplementationInNamespaceCheck>(
         "llvmlibc-implementation-in-namespace");
+    CheckFactories.registerCheck<InlineFunctionDeclCheck>(
+        "llvmlibc-inline-function-decl-check");
     CheckFactories.registerCheck<RestrictSystemLibcHeadersCheck>(
         "llvmlibc-restrict-system-libc-headers");
   }
diff --git a/clang-tools-extra/docs/clang-tidy/checks/llvmlibc/inline-function-decl-check.rst b/clang-tools-extra/docs/clang-tidy/checks/llvmlibc/inline-function-decl-check.rst
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/llvmlibc/inline-function-decl-check.rst
@@ -0,0 +1,8 @@
+.. title:: clang-tidy - llvmlibc-inline-funciton-decl-check
+
+llvmlibc-inline-function-decl-check
+====================================
+
+Checks that all implicit and explicit inline functions in header files are
+tagged with the ``LIBC_INLINE`` macro. See the `libc style guide
+<https://libc.llvm.org/code_style.html>`_ for more information about this macro.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.h b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.h
@@ -0,0 +1,56 @@
+#ifndef LLVM_CLANG_TOOLS_EXTRA_TEST_CLANG_TIDY_CHECKERS_LLVMLIBC_INLINEFUNCTIONDECL_H
+#define LLVM_CLANG_TOOLS_EXTRA_TEST_CLANG_TIDY_CHECKERS_LLVMLIBC_INLINEFUNCTIONDECL_H
+
+#define LIBC_INLINE inline
+
+namespace __llvm_libc {
+
+LIBC_INLINE int addi(int a, int b) {
+  return a + b;
+}
+
+LIBC_INLINE constexpr long addl(long a, long b) {
+  return a + b;
+}
+
+constexpr long long addll(long long a, long long b) {
+  return a + b;
+}
+
+inline unsigned long addul(unsigned long a, unsigned long b) {
+  return a + b;
+}
+
+class  MyClass {
+  int A;
+public:
+  MyClass() : A(123) {}
+
+  LIBC_INLINE MyClass(int V) : A(V) {}
+
+  constexpr operator int() const { return A; }
+
+  LIBC_INLINE bool operator==(const MyClass &RHS) {
+    return RHS.A == A;
+  }
+
+  static int getVal(const MyClass &V) {
+    return V.A;
+  }
+
+  LIBC_INLINE static void setVal(MyClass &V, int A) {
+    V.A = A;
+  }
+
+  constexpr static int addInt(MyClass &V, int A) {
+    return V.A += A;
+  }
+
+  static LIBC_INLINE int mulInt(MyClass &V, int A) {
+    return V.A *= A;
+  }
+};
+
+} // namespace __llvm_libc
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_TEST_CLANG_TIDY_CHECKERS_LLVMLIBC_INLINEFUNCTIONDECL_H
diff --git a/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.cpp b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/llvmlibc/inline-function-decl.cpp
@@ -0,0 +1,25 @@
+// RUN: %check_clang_tidy %s llvmlibc-inline-function-decl-check %t \
+// RUN:   -- -header-filter=.* -- -I %S
+
+#include "inline-function-decl.h"
+
+namespace __llvm_libc {
+
+// Inline functions in .cpp files need not use the LIBC_INLINE tag.
+inline float addf(float a, float b) {
+  return a + b;
+}
+
+// Non-inline functions should not trigger any warning.
+int mul(int a, int b) {
+  return addf(a * b, b);
+}
+
+// CHECK-MESSAGES: warning: 'addll' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'addul' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'MyClass' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'operator int' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'getVal' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'addInt' must be tagged with the LIBC_INLINE macro
+// CHECK-MESSAGES: warning: 'mulInt' must be tagged with the LIBC_INLINE macro
+} // namespace __llvm_libc