Index: clang-tidy/objc/CMakeLists.txt
===================================================================
--- clang-tidy/objc/CMakeLists.txt
+++ clang-tidy/objc/CMakeLists.txt
@@ -4,6 +4,7 @@
   AvoidNSErrorInitCheck.cpp
   AvoidSpinlockCheck.cpp
   ForbiddenSubclassingCheck.cpp
+  IvarDeclarationCheck.cpp
   ObjCTidyModule.cpp
   PropertyDeclarationCheck.cpp
 
Index: clang-tidy/objc/IvarDeclarationCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/objc/IvarDeclarationCheck.h
@@ -0,0 +1,35 @@
+//===--- IvarDeclarationCheck.h - clang-tidy---------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_IVARDECLARATIONCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_IVARDECLARATIONCHECK_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace objc {
+
+/// Finds Objective-C ivars which do not have '_' prefix.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/objc-ivar-declaration.html
+class IvarDeclarationCheck : public ClangTidyCheck {
+public:
+  IvarDeclarationCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace objc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_IVARDECLARATIONCHECK_H
Index: clang-tidy/objc/IvarDeclarationCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/objc/IvarDeclarationCheck.cpp
@@ -0,0 +1,52 @@
+//===--- IvarDeclarationCheck.cpp - clang-tidy-----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "IvarDeclarationCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "llvm/Support/Regex.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace objc {
+
+namespace {
+
+FixItHint generateFixItHint(const ObjCIvarDecl *Decl) {
+  auto IvarName = Decl->getName();
+  if (IvarName[0] != '_') {
+    auto NewName = Decl->getName().str();
+    NewName.insert(0, 1, '_');
+    return FixItHint::CreateReplacement(
+        CharSourceRange::getTokenRange(SourceRange(Decl->getLocation())),
+        llvm::StringRef(NewName));
+  }
+  return FixItHint();
+}
+}  // namespace
+
+void IvarDeclarationCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      objcIvarDecl(unless(matchesName("::_[a-zA-Z]+[a-zA-Z0-9_]*")))
+          .bind("ivar"),
+      this);
+}
+
+void IvarDeclarationCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *MatchedDecl = Result.Nodes.getNodeAs<ObjCIvarDecl>("ivar");
+  diag(MatchedDecl->getLocation(),
+       "instance variable %0 not using '_' as prefix")
+      << MatchedDecl << generateFixItHint(MatchedDecl);
+}
+
+}  // namespace objc
+}  // namespace tidy
+}  // namespace clang
Index: clang-tidy/objc/ObjCTidyModule.cpp
===================================================================
--- clang-tidy/objc/ObjCTidyModule.cpp
+++ clang-tidy/objc/ObjCTidyModule.cpp
@@ -13,6 +13,7 @@
 #include "AvoidNSErrorInitCheck.h"
 #include "AvoidSpinlockCheck.h"
 #include "ForbiddenSubclassingCheck.h"
+#include "IvarDeclarationCheck.h"
 #include "PropertyDeclarationCheck.h"
 
 using namespace clang::ast_matchers;
@@ -30,6 +31,8 @@
         "objc-avoid-spinlock");
     CheckFactories.registerCheck<ForbiddenSubclassingCheck>(
         "objc-forbidden-subclassing");
+    CheckFactories.registerCheck<IvarDeclarationCheck>(
+        "objc-ivar-declaration");
     CheckFactories.registerCheck<PropertyDeclarationCheck>(
         "objc-property-declaration");
   }
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -57,6 +57,11 @@
 Improvements to clang-tidy
 --------------------------
 
+- New :doc:`objc-ivar-declaration
+  <clang-tidy/checks/objc-ivar-declaration>` check
+
+  New check that finds Objective-C ivars that do not have a '_' prefix.
+
 - New module `abseil` for checks related to the `Abseil <https://abseil.io>`_
   library.
 
Index: docs/clang-tidy/checks/list.rst
===================================================================
--- docs/clang-tidy/checks/list.rst
+++ docs/clang-tidy/checks/list.rst
@@ -187,6 +187,7 @@
    objc-avoid-nserror-init
    objc-avoid-spinlock
    objc-forbidden-subclassing
+   objc-ivar-declaration
    objc-property-declaration
    performance-faster-string-find
    performance-for-range-copy
Index: docs/clang-tidy/checks/objc-ivar-declaration.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/objc-ivar-declaration.rst
@@ -0,0 +1,21 @@
+.. title:: clang-tidy - objc-ivar-declaration
+
+objc-ivar-declaration
+=====================
+
+Finds ivar declarations in Objective-C files that do not follow the pattern
+in Apple's programming guide. The ivar name should be fixed with '_'.
+
+For code of ivar declaration:
+
+.. code-block:: objc
+
+   int barWithoutPrefix;
+
+The fix will be:
+
+.. code-block:: objc
+
+   int _barWithoutPrefix;
+
+The corresponding style rule: https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingIvarsAndTypes.html#//apple_ref/doc/uid/20001284-1001757
\ No newline at end of file
Index: test/clang-tidy/objc-ivar-declaration.m
===================================================================
--- /dev/null
+++ test/clang-tidy/objc-ivar-declaration.m
@@ -0,0 +1,11 @@
+// RUN: %check_clang_tidy %s objc-ivar-declaration %t
+@interface Foo
+@end 
+
+@interface Foo () {
+    int _bar;
+    int barWithoutPrefix;
+    // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: instance variable 'barWithoutPrefix' not using '_' as prefix [objc-ivar-declaration]
+    // CHECK-FIXES: int _barWithoutPrefix;
+}
+@end