diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -101,6 +101,18 @@
   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 +135,12 @@
     if (!isPositive)
       remarkOpt = remarkOpt.substr(3);
 
+    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.f90 b/flang/test/Driver/optimization-remark.f90
--- a/flang/test/Driver/optimization-remark.f90
+++ b/flang/test/Driver/optimization-remark.f90
@@ -11,6 +11,12 @@
 ! RUN: %flang %s -O1 -Rno-pass-missed 2>&1 | FileCheck %s --allow-empty --check-prefix=NO-REMARKS
 ! RUN: %flang %s -O1 -Rno-pass-analysis 2>&1 | FileCheck %s --allow-empty --check-prefix=NO-REMARKS
 
+! Check "unknown remark option" warning
+! RUN: %flang %s -O1 -R 2>&1 | FileCheck %s --check-prefix=WARN
+
+! Check "unknown remark option" warning with suggestion
+! RUN: %flang %s -O1 -Rpas 2>&1 | FileCheck %s --check-prefix=WARN-SUGGEST
+
 ! Check full -Rpass message is emitted
 ! RUN: %flang %s -O1 -Rpass 2>&1 | FileCheck %s
 
@@ -21,13 +27,15 @@
 ! RUN: %flang %s -O1 -Rpass-analysis 2>&1 | FileCheck %s --check-prefix=REMARKS-ANALYSIS
 
 ! CHECK: remark: Loop deleted because it is invariant
+! REMARKS: remark:
+! NO-REMARKS-NOT: remark:
 ! REMARKS-MISSED: remark: loop not vectorized
 ! REMARKS-MISSED-NOT: instruction cannot be vectorized
 ! REMARKS-ANALYSIS: remark: loop not vectorized: instruction cannot be vectorized
 ! REMARKS-ANALYSIS-NOT: will not be inlined into {{.*}} because its definition is unavailable
 
-! REMARKS: remark:
-! NO-REMARKS-NOT: remark:
+! WARN: warning: unknown remark option '-R'
+! WARN-SUGGEST: warning: unknown remark option '-Rpas'; did you mean '-Rpass'?
 
 program forttest
     implicit none