Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -740,7 +740,7 @@
   MarshallingInfoStringVector<DiagnosticOpts<"UndefPrefixes">>;
 def Wwrite_strings : Flag<["-"], "Wwrite-strings">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def Wno_write_strings : Flag<["-"], "Wno-write-strings">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
-def W_Joined : Joined<["-"], "W">, Group<W_Group>, Flags<[CC1Option, CoreOption]>,
+def W_Joined : Joined<["-"], "W">, Group<W_Group>, Flags<[CC1Option, CoreOption, FC1Option, FlangOption]>,
   MetaVarName<"<warning>">, HelpText<"Enable the specified warning">;
 def Xanalyzer : Separate<["-"], "Xanalyzer">,
   HelpText<"Pass <arg> to the static analyzer">, MetaVarName<"<arg>">,
Index: clang/lib/Driver/ToolChains/Flang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Flang.cpp
+++ clang/lib/Driver/ToolChains/Flang.cpp
@@ -42,7 +42,8 @@
 
 void Flang::AddOtherOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
   Args.AddAllArgs(CmdArgs,
-                  {options::OPT_module_dir, options::OPT_fdebug_module_writer});
+                  {options::OPT_module_dir, options::OPT_fdebug_module_writer,
+                   options::OPT_W_Joined});
 }
 
 void Flang::ConstructJob(Compilation &C, const JobAction &JA,
Index: flang/include/flang/Frontend/CompilerInvocation.h
===================================================================
--- flang/include/flang/Frontend/CompilerInvocation.h
+++ flang/include/flang/Frontend/CompilerInvocation.h
@@ -71,6 +71,8 @@
 
   bool debugModuleDir_ = false;
 
+  bool warnAsErr_ = false;
+
   // Fortran Dialect options
   Fortran::common::IntrinsicTypeDefaultKinds defaultKinds_;
 
@@ -96,6 +98,9 @@
   bool &debugModuleDir() { return debugModuleDir_; }
   const bool &debugModuleDir() const { return debugModuleDir_; }
 
+  bool &warnAsErr() { return warnAsErr_; }
+  const bool &warnAsErr() const { return warnAsErr_; }
+
   Fortran::common::IntrinsicTypeDefaultKinds &defaultKinds() {
     return defaultKinds_;
   }
@@ -116,6 +121,8 @@
 
   void SetDebugModuleDir(bool flag) { debugModuleDir_ = flag; }
 
+  void SetWarnAsErr(bool flag) { warnAsErr_ = flag; }
+
   /// Set the Fortran options to predifined defaults. These defaults are
   /// consistend with f18/f18.cpp.
   // TODO: We should map frontendOpts_ to parserOpts_ instead. For that, we
Index: flang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- flang/lib/Frontend/CompilerInvocation.cpp
+++ flang/lib/Frontend/CompilerInvocation.cpp
@@ -327,6 +327,11 @@
   if (args.hasArg(clang::driver::options::OPT_fdebug_module_writer)) {
     res.SetDebugModuleDir(true);
   }
+
+  // -Werror option
+  if (args.hasArg(clang::driver::options::OPT_W_Joined)) {
+    res.SetWarnAsErr(true);
+  }
 }
 
 /// Parses all Dialect related arguments and populates the variables
@@ -518,5 +523,6 @@
       defaultKinds(), fortranOptions.features, allCookedSources);
 
   semanticsContext_->set_moduleDirectory(moduleDir())
-      .set_searchDirectories(fortranOptions.searchDirectories);
+      .set_searchDirectories(fortranOptions.searchDirectories)
+      .set_warningsAreErrors(warnAsErr());
 }
Index: flang/test/Driver/driver-help-hidden.f90
===================================================================
--- flang/test/Driver/driver-help-hidden.f90
+++ flang/test/Driver/driver-help-hidden.f90
@@ -47,6 +47,7 @@
 ! CHECK-NEXT: -o <file> Write output to <file>
 ! CHECK-NEXT: -U <macro>             Undefine macro <macro>
 ! CHECK-NEXT: --version Print version information
+! CHECK-NEXT: -W<warning>            Enable the specified warning
 ! CHECK-NEXT: -Xflang <arg>          Pass <arg> to the flang compiler
 
 !-------------------------------------------------------------
Index: flang/test/Driver/driver-help.f90
===================================================================
--- flang/test/Driver/driver-help.f90
+++ flang/test/Driver/driver-help.f90
@@ -47,6 +47,7 @@
 ! HELP-NEXT: -o <file>              Write output to <file>
 ! HELP-NEXT: -U <macro>             Undefine macro <macro>
 ! HELP-NEXT: --version              Print version information
+! HELP-NEXT: -W<warning>            Enable the specified warning
 ! HELP-NEXT: -Xflang <arg>          Pass <arg> to the flang compiler
 
 !-------------------------------------------------------------
@@ -94,6 +95,7 @@
 ! HELP-FC1-NEXT: -test-io               Run the InputOuputTest action. Use for development and testing only.
 ! HELP-FC1-NEXT: -U <macro>             Undefine macro <macro>
 ! HELP-FC1-NEXT: --version              Print version information
+! HELP-FC1-NEXT: -W<warning>            Enable the specified warning
 
 !---------------
 ! EXPECTED ERROR
Index: flang/test/Semantics/dosemantics03.f90
===================================================================
--- flang/test/Semantics/dosemantics03.f90
+++ flang/test/Semantics/dosemantics03.f90
@@ -1,4 +1,5 @@
 ! RUN: %S/test_errors.sh %s %t %f18 -Mstandard -Werror
+! RUN: %S/test_errors.sh %s %t %flang_fc1 -Werror
 
 ! Issue 458 -- semantic checks for a normal DO loop.  The DO variable
 ! and the initial, final, and step expressions must be INTEGER if the