diff --git a/clang/include/clang/Tooling/Tooling.h b/clang/include/clang/Tooling/Tooling.h
--- a/clang/include/clang/Tooling/Tooling.h
+++ b/clang/include/clang/Tooling/Tooling.h
@@ -268,11 +268,17 @@
 
   ~ToolInvocation();
 
-  /// Set a \c DiagnosticConsumer to use during parsing.
+  /// Set a \c DiagnosticConsumer to use during driver command-line parsing and
+  /// the action invocation itself.
   void setDiagnosticConsumer(DiagnosticConsumer *DiagConsumer) {
     this->DiagConsumer = DiagConsumer;
   }
 
+  /// Set a \c DiagnosticOptions to use during driver command-line parsing.
+  void setDiagnosticOptions(DiagnosticOptions *DiagOpts) {
+    this->DiagOpts = DiagOpts;
+  }
+
   /// Run the clang invocation.
   ///
   /// \returns True if there were no errors during execution.
@@ -290,6 +296,7 @@
   FileManager *Files;
   std::shared_ptr<PCHContainerOperations> PCHContainerOps;
   DiagnosticConsumer *DiagConsumer = nullptr;
+  DiagnosticOptions *DiagOpts = nullptr;
 };
 
 /// Utility to run a FrontendAction over a set of files.
diff --git a/clang/lib/Tooling/Tooling.cpp b/clang/lib/Tooling/Tooling.cpp
--- a/clang/lib/Tooling/Tooling.cpp
+++ b/clang/lib/Tooling/Tooling.cpp
@@ -343,10 +343,17 @@
   for (const std::string &Str : CommandLine)
     Argv.push_back(Str.c_str());
   const char *const BinaryName = Argv[0];
-  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts =
-      CreateAndPopulateDiagOpts(Argv);
-  TextDiagnosticPrinter DiagnosticPrinter(
-      llvm::errs(), &*DiagOpts);
+
+  // Parse diagnostic options from the driver command-line only if none were
+  // explicitly set.
+  IntrusiveRefCntPtr<DiagnosticOptions> ParsedDiagOpts;
+  DiagnosticOptions *DiagOpts = this->DiagOpts;
+  if (!DiagOpts) {
+    ParsedDiagOpts = CreateAndPopulateDiagOpts(Argv);
+    DiagOpts = &*ParsedDiagOpts;
+  }
+
+  TextDiagnosticPrinter DiagnosticPrinter(llvm::errs(), DiagOpts);
   IntrusiveRefCntPtr<DiagnosticsEngine> Diagnostics =
       CompilerInstance::createDiagnostics(
           &*DiagOpts, DiagConsumer ? DiagConsumer : &DiagnosticPrinter, false);
diff --git a/clang/unittests/Tooling/ToolingTest.cpp b/clang/unittests/Tooling/ToolingTest.cpp
--- a/clang/unittests/Tooling/ToolingTest.cpp
+++ b/clang/unittests/Tooling/ToolingTest.cpp
@@ -265,6 +265,41 @@
   EXPECT_EQ(Consumer.NumErrorsSeen, 0u);
 }
 
+TEST(ToolInvocation, CustomDiagnosticOptionsOverwriteParsedOnes) {
+  llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> OverlayFileSystem(
+      new llvm::vfs::OverlayFileSystem(llvm::vfs::getRealFileSystem()));
+  llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
+      new llvm::vfs::InMemoryFileSystem);
+  OverlayFileSystem->pushOverlay(InMemoryFileSystem);
+  llvm::IntrusiveRefCntPtr<FileManager> Files(
+      new FileManager(FileSystemOptions(), OverlayFileSystem));
+
+  std::vector<std::string> Args;
+  Args.push_back("tool-executable");
+  Args.push_back("-target");
+  // Invalid argument that by default results in an error diagnostic:
+  Args.push_back("i386-apple-ios14.0-simulator");
+  // Argument that downgrades the error into a warning:
+  Args.push_back("-Wno-error=invalid-ios-deployment-target");
+  Args.push_back("-fsyntax-only");
+  Args.push_back("test.cpp");
+
+  clang::tooling::ToolInvocation Invocation(
+      Args, std::make_unique<SyntaxOnlyAction>(), Files.get());
+  InMemoryFileSystem->addFile(
+      "test.cpp", 0, llvm::MemoryBuffer::getMemBuffer("int a() {}\n"));
+  ErrorCountingDiagnosticConsumer Consumer;
+  Invocation.setDiagnosticConsumer(&Consumer);
+
+  auto DiagOpts = llvm::makeIntrusiveRefCnt<DiagnosticOptions>();
+  Invocation.setDiagnosticOptions(&*DiagOpts);
+
+  EXPECT_TRUE(Invocation.run());
+  // Check that `-Wno-error=invalid-ios-deployment-target` didn't downgrade the
+  // error into a warning due to being overwritten by custom diagnostic options.
+  EXPECT_EQ(Consumer.NumErrorsSeen, 1u);
+}
+
 struct DiagnosticConsumerExpectingSourceManager : public DiagnosticConsumer {
   bool SawSourceManager;