diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -194,6 +194,9 @@
 def err_drv_I_dash_not_supported : Error<
   "'%0' not supported, please use -iquote instead">;
 def err_drv_unknown_argument : Error<"unknown argument: '%0'">;
+def warn_drv_flang_argument : Warning<
+  "command line option '%0' is only valid in Flang mode (i.e. for Fortran input)">,
+  InGroup<UnknownArgument>;
 def err_drv_unknown_argument_with_suggestion : Error<
   "unknown argument '%0'; did you mean '%1'?">;
 def warn_drv_unknown_argument_clang_cl : Warning<
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -262,20 +262,44 @@
   }
 
   for (const Arg *A : Args.filtered(options::OPT_UNKNOWN)) {
-    unsigned DiagID;
+    // The default diagnostic when the option is unknown
+    unsigned DiagID = diag::err_drv_unknown_argument;
+
+    // Try to find a good hint for the user
     auto ArgString = A->getAsString(Args);
     std::string Nearest;
-    if (getOpts().findNearest(
-          ArgString, Nearest, IncludedFlagsBitmask, ExcludedFlagsBitmask) > 1) {
-      DiagID = IsCLMode() ? diag::warn_drv_unknown_argument_clang_cl
-                          : diag::err_drv_unknown_argument;
-      Diags.Report(DiagID) << ArgString;
-    } else {
+    unsigned Distance = getOpts().findNearest(
+        ArgString, Nearest, IncludedFlagsBitmask, ExcludedFlagsBitmask);
+    assert(Distance != 0 && "This option should not be 'unknown'");
+
+    if (Distance == 1) {
+      // Found a good suggestion - propose that through a diagnostic
       DiagID = IsCLMode()
                    ? diag::warn_drv_unknown_argument_clang_cl_with_suggestion
                    : diag::err_drv_unknown_argument_with_suggestion;
       Diags.Report(DiagID) << ArgString << Nearest;
+    } else {
+      // No good suggestion was found - issue an error (default) or a warning.
+      // The actual diagnostic depends on the mode in which the driver operates
+      if (IsCLMode())
+        // In CL mode just warn the user (MSVC consumes everything anyway)
+        DiagID = diag::warn_drv_unknown_argument_clang_cl;
+      else if (!IsFlangMode()) {
+        // In non-Flang mode, warn _if_ this is a Flang-only flag. Clang will
+        // reject/ignore these, but like in GCC, this shouldn't be an error.
+        // TODO: Currently this will only work for non-alias options. Options
+        // that are aliases are internally stored as their non-alias versions.
+        // When mixed with `OPT_UNKNOWN`, this seems to confuse `findNearest`.
+        unsigned ExcludedFlagsBitmaskWithFlang =
+            ExcludedFlagsBitmask & ~options::FlangOnlyOption;
+        if (getOpts().findNearest(ArgString, Nearest, IncludedFlagsBitmask,
+                                  ExcludedFlagsBitmaskWithFlang) == 0)
+          DiagID = diag::warn_drv_flang_argument;
+      } // TODO: In Flang-mode, check whether this is a Clang-only flag.
+
+      Diags.Report(DiagID) << ArgString;
     }
+
     ContainsError |= Diags.getDiagnosticLevel(DiagID, SourceLocation()) >
                      DiagnosticsEngine::Warning;
   }
diff --git a/clang/test/Driver/flang-only-option-diags.c b/clang/test/Driver/flang-only-option-diags.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/flang-only-option-diags.c
@@ -0,0 +1,28 @@
+// Test the behaviour of clang with a `FlangOnlyOption` and various diagnostics
+// flags. We want to make sure that users can control whether Flang-only options
+
+// RUN: %clang -fsyntax-only -ffree-form %s 2>&1 | FileCheck %s --check-prefix=WARNING
+// RUN: not %clang -fsyntax-only -Werror -ffree-form %s 2>&1 | FileCheck %s --check-prefix=ERROR
+// RUN: %clang -fsyntax-only -Werror -Wno-unknown-argument -Wno-unused-command-line-argument -ffree-form %s 2>&1 | FileCheck %s --allow-empty --check-prefix=NO-DIAGS
+
+//---------------------------
+// EXPECTED OUTPUT - WARNING
+//---------------------------
+// WARNING: clang-{{[0-9]+}}: warning: command line option '-ffree-form' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// WARNING-NEXT: clang-{{[0-9]+}}: warning: argument unused during compilation: '-ffree-form' [-Wunused-command-line-argument]
+
+//------------------------
+// EXPECTED OUTPUT - ERROR
+//------------------------
+// ERROR: clang-13: error: command line option '-ffree-form' is only valid in Flang mode (i.e. for Fortran input) [-Werror,-Wunknown-argument]
+
+//---------------------------------
+// EXPECTED OUTPUT - NO DIAGNOSTICS
+//---------------------------------
+// NO-DIAGS-NOT: error
+// NO-DIAGS-NOT: warning
+
+//-------
+// INPUT
+//-------
+int a;
diff --git a/clang/test/Driver/flang-only-options.c b/clang/test/Driver/flang-only-options.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/flang-only-options.c
@@ -0,0 +1,70 @@
+// Verifies that Flang options marked as `FlangOnlyOption` are corectly recognised and warned about by Clang.
+//
+// TODO: Add aliases (once working)
+
+// RUN: %clang -fsyntax-only \
+// SKIP:  -Xflang \ This option is currently parsed by Clang as OPT_X_Flang (i.e. `-X flang` rather than `-Xflang`)
+// RUN:   -ffree-form \
+// RUN:   -ffixed-form \
+// RUN:   -module-dir \
+// RUN:   -ffixed-line-length=1 \
+// RUN:   -fopenacc \
+// RUN:   -fdefault-double-8 \
+// RUN:   -fdefault-integer-8 \
+// RUN:   -fdefault-real-8 \
+// RUN:   -flarge-sizes \
+// RUN:   -fbackslash \
+// RUN:   -fno-backslash \
+// RUN:   -fxor-operator \
+// RUN:   -fno-xor-operator \
+// RUN:   -flogical-abbreviations \
+// RUN:   -fno-logical-abbreviations \
+// RUN:   -fimplicit-none \
+// RUN:   -fno-implicit-none \
+// RUN:   -falternative-parameter-statement \
+// RUN:   -fintrinsic-modules-path \
+// RUN:   -test-io \
+// RUN:   -fdebug-unparse \
+// RUN:   -fdebug-unparse-with-symbols \
+// RUN:   -fdebug-dump-symbols \
+// RUN:   -fdebug-dump-parse-tree \
+// RUN:   -fdebug-dump-provenance \
+// RUN:   -fdebug-dump-parsing-log \
+// RUN:   -fdebug-measure-parse-tree \
+// RUN:   -fdebug-pre-fir-tree \
+// RUN:   -fdebug-module-writer \
+// RUN:   -fget-symbols-sources \
+// RUN: %s 2>&1 | FileCheck %s
+
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-ffree-form' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-ffixed-form' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-module-dir' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-ffixed-line-length=1' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fopenacc' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdefault-double-8' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdefault-integer-8' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdefault-real-8' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-flarge-sizes' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fbackslash' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fno-backslash' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fxor-operator' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fno-xor-operator' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-flogical-abbreviations' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fno-logical-abbreviations' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fimplicit-none' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fno-implicit-none' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-falternative-parameter-statement' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fintrinsic-modules-path' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-test-io' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-unparse' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-unparse-with-symbols' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-dump-symbols' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-dump-parse-tree' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-dump-provenance' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-dump-parsing-log' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-measure-parse-tree' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-pre-fir-tree' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fdebug-module-writer' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+// CHECK: clang-{{[0-9]+}}: warning: command line option '-fget-symbols-sources' is only valid in Flang mode (i.e. for Fortran input) [-Wunknown-argument]
+
+int a;