diff --git a/clang-tools-extra/clangd/ClangdUnit.cpp b/clang-tools-extra/clangd/ClangdUnit.cpp
--- a/clang-tools-extra/clangd/ClangdUnit.cpp
+++ b/clang-tools-extra/clangd/ClangdUnit.cpp
@@ -113,12 +113,12 @@
   }
 
   void EndOfMainFile() {
-    for (const auto& Entry : MainFileMacros)
+    for (const auto &Entry : MainFileMacros)
       Out->push_back(Entry.getKey());
     llvm::sort(*Out);
   }
 
- private:
+private:
   const SourceManager &SM;
   bool InMainFile = true;
   llvm::StringSet<> MainFileMacros;
@@ -275,12 +275,37 @@
   const LangOptions &LangOpts;
 };
 
+// This is a copy of ClangTidyContext::CachedGlobList.
+// FIXME: Factor out to a common place.
+class CachedGlobList {
+public:
+  CachedGlobList(StringRef Globs) : Globs(Globs) {}
+
+  bool contains(StringRef S) {
+    switch (auto &Result = Cache[S]) {
+    case Yes:
+      return true;
+    case No:
+      return false;
+    case None:
+      Result = Globs.contains(S) ? Yes : No;
+      return Result == Yes;
+    }
+    llvm_unreachable("invalid enum");
+  }
+
+private:
+  tidy::GlobList Globs;
+  enum Tristate { None, Yes, No };
+  llvm::StringMap<Tristate> Cache;
+};
+
 // A wrapper around StoreDiags to handle suppression comments for
 // clang-tidy diagnostics (and possibly other clang-tidy customizations in the
 // future).
 class ClangdDiagnosticConsumer : public StoreDiags {
 public:
-  ClangdDiagnosticConsumer() {
+  ClangdDiagnosticConsumer(llvm::Optional<std::string> WarningsAsErrors) {
     filterDiagnostics([this](DiagnosticsEngine::Level DiagLevel,
                              const clang::Diagnostic &Info,
                              bool IsInsideMainFile) {
@@ -299,14 +324,36 @@
       }
       return true;
     });
+
+    if (WarningsAsErrors) {
+      WarningAsErrorFilter =
+          llvm::make_unique<CachedGlobList>(*WarningsAsErrors);
+    }
   }
 
   void setClangTidyContext(tidy::ClangTidyContext *CTContext) {
     this->CTContext = CTContext;
   }
 
+  void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                        const clang::Diagnostic &Info) override {
+    DiagnosticsEngine::Level Level = DiagLevel;
+
+    // Check for warning-as-error.
+    if (CTContext && WarningAsErrorFilter &&
+        DiagLevel == DiagnosticsEngine::Warning) {
+      std::string CheckName = CTContext->getCheckName(Info.getID());
+      if (!CheckName.empty() && WarningAsErrorFilter->contains(CheckName)) {
+        Level = DiagnosticsEngine::Error;
+      }
+    }
+
+    StoreDiags::HandleDiagnostic(Level, Info);
+  }
+
 private:
   tidy::ClangTidyContext *CTContext = nullptr;
+  std::unique_ptr<CachedGlobList> WarningAsErrorFilter;
 };
 
 } // namespace
@@ -328,7 +375,7 @@
   const PrecompiledPreamble *PreamblePCH =
       Preamble ? &Preamble->Preamble : nullptr;
 
-  ClangdDiagnosticConsumer ASTDiags;
+  ClangdDiagnosticConsumer ASTDiags{Opts.ClangTidyOpts.WarningsAsErrors};
   std::string Content = Buffer->getBuffer();
 
   auto Clang = prepareCompilerInstance(std::move(CI), PreamblePCH,