diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index d16aff84f445f..bc09dec17b7ae 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -101,6 +101,18 @@ createFrontendAction(CompilerInstance &ci) {
   llvm_unreachable("Invalid program action!");
 }
 
+static void emitUnknownDiagWarning(clang::DiagnosticsEngine &diags,
+                                   clang::diag::Flavor flavor,
+                                   llvm::StringRef prefix,
+                                   llvm::StringRef opt) {
+  llvm::StringRef suggestion =
+      clang::DiagnosticIDs::getNearestOption(flavor, opt);
+  diags.Report(clang::diag::warn_unknown_diag_option)
+      << (flavor == clang::diag::Flavor::WarningOrError ? 0 : 1)
+      << (prefix.str() += std::string(opt)) << !suggestion.empty()
+      << (prefix.str() += std::string(suggestion));
+}
+
 // Remarks are ignored by default in Diagnostic.td, hence, we have to
 // enable them here before execution. Clang follows same idea using
 // ProcessWarningOptions in Warnings.cpp
@@ -123,6 +135,13 @@ updateDiagEngineForOptRemarks(clang::DiagnosticsEngine &diagsEng,
     if (!isPositive)
       remarkOpt = remarkOpt.substr(3);
 
+    // Verify that this is a valid optimization remarks option
+    if (diagIDs->getDiagnosticsInGroup(flavor, remarkOpt, diags)) {
+      emitUnknownDiagWarning(diagsEng, flavor, isPositive ? "-R" : "-Rno-",
+                             remarkOpt);
+      return;
+    }
+
     diagsEng.setSeverityForGroup(flavor, remarkOpt,
                                  isPositive ? clang::diag::Severity::Remark
                                             : clang::diag::Severity::Ignored);
diff --git a/flang/test/Driver/optimization-remark-invalid.f90 b/flang/test/Driver/optimization-remark-invalid.f90
index 829ccba895199..a57be4ec3e729 100644
--- a/flang/test/Driver/optimization-remark-invalid.f90
+++ b/flang/test/Driver/optimization-remark-invalid.f90
@@ -5,8 +5,15 @@
 ! Check error on invalid regex -Rpass message is emitted
 ! RUN: not %flang %s -O1 -Rpass=[ -c 2>&1 | FileCheck %s --check-prefix=REGEX-INVALID
 
+! Check "unknown remark option" warning
+! RUN: %flang %s -O1 -R -c 2>&1 | FileCheck %s --check-prefix=WARN
+
+! Check "unknown remark option" warning with suggestion
+! RUN: %flang %s -O1 -Rpas -c 2>&1 | FileCheck %s --check-prefix=WARN-SUGGEST
 
 ! REGEX-INVALID: error: in pattern '-Rpass=[': brackets ([ ]) not balanced
+! WARN: warning: unknown remark option '-R'
+! WARN-SUGGEST: warning: unknown remark option '-Rpas'; did you mean '-Rpass'?
 
 program forttest
 end program forttest