diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -311,7 +311,63 @@
                         : "unknown error");
     return None;
   }
-  if (!PreserveDiags) {
+  tidy::ClangTidyOptions ClangTidyOpts;
+  if (PreserveDiags) {
+    trace::Span Tracer("ClangTidyOpts");
+    ClangTidyOpts = getTidyOptionsForFile(Inputs.ClangTidyProvider, Filename);
+    dlog("ClangTidy configuration for file {0}: {1}", Filename,
+         tidy::configurationAsText(ClangTidyOpts));
+
+    // If clang-tidy is configured to emit clang warnings, we should too.
+    //
+    // Such clang-tidy configuration consists of two parts:
+    //   - ExtraArgs: ["-Wfoo"] causes clang to produce the warnings
+    //   - Checks: "clang-diagnostic-foo" prevents clang-tidy filtering them out
+    //
+    // We treat these as clang warnings, so the Checks part is not relevant.
+    // We must enable the warnings specified in ExtraArgs.
+    //
+    // We *don't* want to change the compile command directly. this can have
+    // too many unexpected effects: breaking the command, interactions with
+    // -- and -Werror, etc. Besides, we've already parsed the command.
+    // Instead we parse the -W<group> flags and handle them directly.
+    auto ProcessWarningOptions = [&](llvm::ArrayRef<std::string> ExtraArgs) {
+      auto &Diags = Clang->getDiagnostics();
+      for (llvm::StringRef Group : ExtraArgs) {
+        // Only handle args that are of the form -W<group>.
+        // Other flags are possible but rare and deliberately out of scope.
+        llvm::SmallVector<diag::kind> Members;
+        if (!Group.consume_front("-W") || Group.empty())
+          continue;
+        if (Diags.getDiagnosticIDs()->getDiagnosticsInGroup(
+                diag::Flavor::WarningOrError, Group, Members))
+          continue;
+
+        // Upgrade the severity of each diagnostic in the group to warning.
+        // If -Werror is on, these will be treated as errors, which we must fix.
+        llvm::DenseSet<diag::kind> NeedsWerrorExclusion;
+        for (diag::kind ID : Members) {
+          if (Diags.getDiagnosticLevel(ID, SourceLocation()) <
+              DiagnosticsEngine::Warning) {
+            Diags.setSeverity(ID, diag::Severity::Warning, SourceLocation());
+            if (Diags.getWarningsAsErrors())
+              NeedsWerrorExclusion.insert(ID);
+          }
+        }
+        if (!NeedsWerrorExclusion.empty()) {
+          // FIXME: there's no API to suppress -Werror for single diagnostics.
+          // In some cases with sub-groups, we may end up erroneously
+          // downgrading diagnostics that were -Werror in the compile command.
+          Diags.setDiagnosticGroupWarningAsError(Group, false);
+        }
+      }
+    };
+
+    if (ClangTidyOpts.ExtraArgs)
+      ProcessWarningOptions(*ClangTidyOpts.ExtraArgs);
+    if (ClangTidyOpts.ExtraArgsBefore)
+      ProcessWarningOptions(*ClangTidyOpts.ExtraArgsBefore);
+  } else {
     // Skips some analysis.
     Clang->getDiagnosticOpts().IgnoreWarnings = true;
   }
@@ -348,10 +404,6 @@
   // diagnostics.
   if (PreserveDiags) {
     trace::Span Tracer("ClangTidyInit");
-    tidy::ClangTidyOptions ClangTidyOpts =
-        getTidyOptionsForFile(Inputs.ClangTidyProvider, Filename);
-    dlog("ClangTidy configuration for file {0}: {1}", Filename,
-         tidy::configurationAsText(ClangTidyOpts));
     tidy::ClangTidyCheckFactories CTFactories;
     for (const auto &E : tidy::ClangTidyModuleRegistry::entries())
       E.instantiate()->addCheckFactories(CTFactories);
diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -517,6 +517,58 @@
           DiagSeverity(DiagnosticsEngine::Error)))));
 }
 
+TidyProvider addClangArgs(std::vector<llvm::StringRef> ExtraArgs) {
+  return [ExtraArgs = std::move(ExtraArgs)](tidy::ClangTidyOptions &Opts,
+                                            llvm::StringRef) {
+    if (!Opts.ExtraArgs)
+      Opts.ExtraArgs.emplace();
+    for (llvm::StringRef Arg : ExtraArgs)
+      Opts.ExtraArgs->emplace_back(Arg);
+  };
+}
+
+TEST(DiagnosticTest, ClangTidyEnablesClangWarning) {
+  Annotations Main(R"cpp( // error-ok
+    static void [[foo]]() {}
+  )cpp");
+  TestTU TU = TestTU::withCode(Main.code());
+  // This is always emitted as a clang warning, not a clang-tidy diagnostic.
+  auto UnusedFooWarning =
+      AllOf(Diag(Main.range(), "unused function 'foo'"),
+            DiagName("-Wunused-function"), DiagSource(Diag::Clang),
+            DiagSeverity(DiagnosticsEngine::Warning));
+
+  // Check the -Wunused warning isn't initially on.
+  EXPECT_THAT(*TU.build().getDiagnostics(), IsEmpty());
+
+  // We enable warnings based on clang-tidy extra args.
+  TU.ClangTidyProvider = addClangArgs({"-Wunused"});
+  EXPECT_THAT(*TU.build().getDiagnostics(), ElementsAre(UnusedFooWarning));
+
+  // But we don't respect other args.
+  TU.ClangTidyProvider = addClangArgs({"-Wunused", "-Dfoo=bar"});
+  EXPECT_THAT(*TU.build().getDiagnostics(), ElementsAre(UnusedFooWarning))
+      << "Not unused function 'bar'!";
+
+  // -Werror doesn't apply to warnings enabled by clang-tidy extra args.
+  TU.ExtraArgs = {"-Werror"};
+  TU.ClangTidyProvider = addClangArgs({"-Wunused"});
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              ElementsAre(DiagSeverity(DiagnosticsEngine::Warning)));
+
+  // But clang-tidy extra args won't *downgrade* errors to warnings either.
+  TU.ExtraArgs = {"-Wunused", "-Werror"};
+  TU.ClangTidyProvider = addClangArgs({"-Wunused"});
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              ElementsAre(DiagSeverity(DiagnosticsEngine::Error)));
+
+  // FIXME: we're erroneously downgrading the whole group, this should be Error.
+  TU.ExtraArgs = {"-Wunused-function", "-Werror"};
+  TU.ClangTidyProvider = addClangArgs({"-Wunused"});
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              ElementsAre(DiagSeverity(DiagnosticsEngine::Warning)));
+}
+
 TEST(DiagnosticTest, LongFixMessages) {
   // We limit the size of printed code.
   Annotations Source(R"cpp(