Index: clang-tidy/readability/NamespaceCommentCheck.h
===================================================================
--- clang-tidy/readability/NamespaceCommentCheck.h
+++ clang-tidy/readability/NamespaceCommentCheck.h
@@ -31,6 +31,7 @@
   void storeOptions(ClangTidyOptions::OptionMap &Options) override;
 
   llvm::Regex NamespaceCommentPattern;
+  const bool ReplaceUnknownComments;
   const unsigned ShortNamespaceLines;
   const unsigned SpacesBeforeComments;
 };
Index: clang-tidy/readability/NamespaceCommentCheck.cpp
===================================================================
--- clang-tidy/readability/NamespaceCommentCheck.cpp
+++ clang-tidy/readability/NamespaceCommentCheck.cpp
@@ -25,10 +25,15 @@
       NamespaceCommentPattern("^/[/*] *(end (of )?)? *(anonymous|unnamed)? *"
                               "namespace( +([a-zA-Z0-9_]+))?\\.? *(\\*/)?$",
                               llvm::Regex::IgnoreCase),
+      ReplaceUnknownComments(Options.get("ReplaceUnknownComments", "false") ==
+                             "true"),
       ShortNamespaceLines(Options.get("ShortNamespaceLines", 1u)),
       SpacesBeforeComments(Options.get("SpacesBeforeComments", 1u)) {}
 
 void NamespaceCommentCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
+  // TODO: Implement (de-)serialization of bool options in the core library.
+  Options.store(Opts, "ReplaceUnknownComments",
+                ReplaceUnknownComments ? "true" : "false");
   Options.store(Opts, "ShortNamespaceLines", ShortNamespaceLines);
   Options.store(Opts, "SpacesBeforeComments", SpacesBeforeComments);
 }
@@ -82,6 +87,9 @@
   // to insert a line break.
   bool NeedLineBreak = NextTokenIsOnSameLine && Tok.isNot(tok::eof);
 
+  SourceRange OldCommentRange(AfterRBrace, AfterRBrace);
+  StringRef Message = "%0 not terminated with a closing comment";
+
   // Try to find existing namespace closing comment on the same line.
   if (Tok.is(tok::comment) && NextTokenIsOnSameLine) {
     StringRef Comment(Sources.getCharacterData(Loc), Tok.getLength());
@@ -101,20 +109,25 @@
 
       // Otherwise we need to fix the comment.
       NeedLineBreak = Comment.startswith("/*");
-      CharSourceRange OldCommentRange = CharSourceRange::getCharRange(
-          SourceRange(Loc, Loc.getLocWithOffset(Tok.getLength())));
-      diag(Loc, "namespace closing comment refers to a wrong namespace '%0'")
-          << NamespaceNameInComment
-          << FixItHint::CreateReplacement(
-                 OldCommentRange, getNamespaceComment(ND, NeedLineBreak));
-      return;
-    }
+      OldCommentRange =
+          SourceRange(AfterRBrace, Loc.getLocWithOffset(Tok.getLength()));
+      Message =
+          (llvm::Twine(
+               "%0 ends with a comment that refers to a wrong namespace '") +
+           NamespaceNameInComment + "'").str();
+    } else if (Comment.startswith("//")) {
+      // Assume that this is an unrecognized form of a namespace closing line
+      // comment. Replace it if needed.
 
-    // This is not a recognized form of a namespace closing comment.
-    // Leave line comment on the same line. Move block comment to the next line,
-    // as it can be multi-line or there may be other tokens behind it.
-    if (Comment.startswith("//"))
       NeedLineBreak = false;
+      if (ReplaceUnknownComments) {
+        OldCommentRange =
+            SourceRange(AfterRBrace, Loc.getLocWithOffset(Tok.getLength()));
+        Message = "%0 ends with an unrecognized comment";
+      }
+    }
+    // If it's a block comment, just move it to the next line, as it can be
+    // multi-line or there may be other tokens behind it.
   }
 
   std::string NamespaceName =
@@ -122,11 +135,11 @@
           ? "anonymous namespace"
           : ("namespace '" + ND->getNameAsString() + "'");
 
-  diag(AfterRBrace, "%0 not terminated with a closing comment")
+  diag(AfterRBrace, Message)
       << NamespaceName
-      << FixItHint::CreateInsertion(AfterRBrace,
-                                    std::string(SpacesBeforeComments, ' ') +
-                                        getNamespaceComment(ND, NeedLineBreak));
+      << FixItHint::CreateReplacement(
+             OldCommentRange, std::string(SpacesBeforeComments, ' ') +
+                                  getNamespaceComment(ND, NeedLineBreak));
   diag(ND->getLocation(), "%0 starts here", DiagnosticIDs::Note)
       << NamespaceName;
 }
Index: unittests/clang-tidy/ClangTidyTest.h
===================================================================
--- unittests/clang-tidy/ClangTidyTest.h
+++ unittests/clang-tidy/ClangTidyTest.h
@@ -42,12 +42,12 @@
 };
 
 template <typename T>
-std::string runCheckOnCode(StringRef Code,
-                           std::vector<ClangTidyError> *Errors = nullptr,
-                           const Twine &Filename = "input.cc",
-                           ArrayRef<std::string> ExtraArgs = None) {
-  ClangTidyOptions Options;
-  Options.Checks = "*";
+std::string
+runCheckWithOptionsOnCode(const ClangTidyOptions &Options, StringRef Code,
+                          std::vector<ClangTidyError> *Errors = nullptr,
+                          const Twine &FileName = "input.cc",
+                          ArrayRef<std::string> ExtraArgs = None) {
+
   ClangTidyContext Context(llvm::make_unique<DefaultOptionsProvider>(
       ClangTidyGlobalOptions(), Options));
   ClangTidyDiagnosticConsumer DiagConsumer(Context);
@@ -59,12 +59,12 @@
   ArgCXX11.push_back("-fsyntax-only");
   ArgCXX11.push_back("-std=c++11");
   ArgCXX11.insert(ArgCXX11.end(), ExtraArgs.begin(), ExtraArgs.end());
-  ArgCXX11.push_back(Filename.str());
+  ArgCXX11.push_back(FileName.str());
   llvm::IntrusiveRefCntPtr<FileManager> Files(
       new FileManager(FileSystemOptions()));
   tooling::ToolInvocation Invocation(
       ArgCXX11, new TestClangTidyAction(Check, Finder, Context), Files.get());
-  Invocation.mapVirtualFile(Filename.str(), Code);
+  Invocation.mapVirtualFile(FileName.str(), Code);
   Invocation.setDiagnosticConsumer(&DiagConsumer);
   if (!Invocation.run())
     return "";
@@ -78,6 +78,17 @@
   return tooling::applyAllReplacements(Code, Fixes);
 }
 
+template <typename T>
+std::string runCheckOnCode(StringRef Code,
+                           std::vector<ClangTidyError> *Errors = nullptr,
+                           const Twine &FileName = "input.cc",
+                           ArrayRef<std::string> ExtraArgs = None) {
+  ClangTidyOptions Options;
+  Options.Checks = "*";
+  return runCheckWithOptionsOnCode<T>(Options, Code, Errors, FileName,
+                                      ExtraArgs);
+}
+
 #define EXPECT_NO_CHANGES(Check, Code)                                         \
   EXPECT_EQ(Code, runCheckOnCode<Check>(Code))
 
Index: unittests/clang-tidy/ReadabilityModuleTest.cpp
===================================================================
--- unittests/clang-tidy/ReadabilityModuleTest.cpp
+++ unittests/clang-tidy/ReadabilityModuleTest.cpp
@@ -75,11 +75,6 @@
             runCheckOnCode<NamespaceCommentCheck>("namespace {\n"
                                                   "}\n"
                                                   "// namespace"));
-  // Leave unknown comments.
-  EXPECT_EQ("namespace {\n"
-            "} // namespace // random text",
-            runCheckOnCode<NamespaceCommentCheck>("namespace {\n"
-                                                  "} // random text"));
 }
 
 TEST(NamespaceCommentCheckTest, FixWrongComments) {
@@ -94,6 +89,26 @@
             "} // namespace",
             runCheckOnCode<NamespaceCommentCheck>("namespace {\n"
                                                   "} // namespace asdf"));
+  ClangTidyOptions DontReplaceUnknownComments;
+  DontReplaceUnknownComments.Checks = "*";
+  DontReplaceUnknownComments.CheckOptions["test-check.ReplaceUnknownComments"] =
+      "false";
+  // By default, don't remove unknown comments.
+  EXPECT_EQ("namespace {\n"
+            "} // namespace // random text",
+            runCheckWithOptionsOnCode<NamespaceCommentCheck>(
+                DontReplaceUnknownComments, "namespace {\n"
+                                            "} // random text"));
+  ClangTidyOptions ReplaceUnknownComments;
+  ReplaceUnknownComments.Checks = "*";
+  ReplaceUnknownComments.CheckOptions["test-check.ReplaceUnknownComments"] =
+      "true";
+  // Remove unknown comments.
+  EXPECT_EQ("namespace {\n"
+            "} // namespace",
+            runCheckWithOptionsOnCode<NamespaceCommentCheck>(
+                ReplaceUnknownComments, "namespace {\n"
+                                        "} // random text"));
 }
 
 TEST(BracesAroundStatementsCheck, IfWithComments) {