Index: clang-tidy/readability/IdentifierNamingCheck.cpp
===================================================================
--- clang-tidy/readability/IdentifierNamingCheck.cpp
+++ clang-tidy/readability/IdentifierNamingCheck.cpp
@@ -109,6 +109,7 @@
     m(TemplateParameter) \
     m(TypeAlias) \
     m(MacroDefinition) \
+    m(ObjcIvar) \
 
 enum StyleKind {
 #define ENUMERATE(v) SK_ ## v,
@@ -384,6 +385,9 @@
     const NamedDecl *D,
     const std::vector<llvm::Optional<IdentifierNamingCheck::NamingStyle>>
         &NamingStyles) {
+  if (isa<ObjCIvarDecl>(D) && NamingStyles[SK_ObjcIvar])
+    return SK_ObjcIvar;
+
   if (isa<TypedefDecl>(D) && NamingStyles[SK_Typedef])
     return SK_Typedef;
 
Index: test/clang-tidy/readability-identifier-naming-objc.m
===================================================================
--- test/clang-tidy/readability-identifier-naming-objc.m
+++ test/clang-tidy/readability-identifier-naming-objc.m
@@ -0,0 +1,15 @@
+// RUN: %check_clang_tidy %s readability-identifier-naming %t \
+// RUN: -config='{CheckOptions: \
+// RUN:  [{key: readability-identifier-naming.ObjcIvarPrefix, value: '_'}]}' \
+// RUN: --
+
+@interface Foo
+@end 
+
+@interface Foo () {
+    int _bar;
+    int barWithoutPrefix;
+    // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: invalid case style for objc ivar 'barWithoutPrefix' [readability-identifier-naming]
+    // CHECK-FIXES: int _barWithoutPrefix;
+}
+@end