diff --git a/include/clang/Basic/DiagnosticDriverKinds.td b/include/clang/Basic/DiagnosticDriverKinds.td
index 529a8da93cb6c851a75d2200026a990b6e6770ab..1ac5cda784474e1334ff76b986a2cd9a969aa467 100644
--- a/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/include/clang/Basic/DiagnosticDriverKinds.td
@@ -106,6 +106,10 @@ def err_drv_malformed_sanitizer_blacklist : Error<
 
 def err_target_unsupported_arch
   : Error<"the target architecture '%0' is not supported by the target '%1'">;
+def err_cpu_unsupported_isa
+  : Error<"CPU '%0' does not support '%1' execution mode">;
+def err_arch_unsupported_isa
+  : Error<"Architecture '%0' does not support '%1' execution mode">;
 
 def err_drv_I_dash_not_supported : Error<
   "'%0' not supported, please use -iquote instead">;
diff --git a/lib/Driver/ToolChain.cpp b/lib/Driver/ToolChain.cpp
index 6c7a6c8794c43af065e068bae3004ab8fe0e1665..1e446dc1ebccb91f2f497f684036c3baca8eb451 100644
--- a/lib/Driver/ToolChain.cpp
+++ b/lib/Driver/ToolChain.cpp
@@ -517,6 +517,18 @@ std::string ToolChain::ComputeLLVMTriple(const ArgList &Args,
     else
       ArchName = "arm";
 
+    // Check if ARM ISA was explicitly selected (using -mno-thumb or -marm) for
+    // M-Class CPUs/architecture variants, which is not supported.
+    bool ARMModeRequested = !Args.hasFlag(options::OPT_mthumb,
+                                          options::OPT_mno_thumb, ThumbDefault);
+    if (IsMProfile && ARMModeRequested) {
+      if (!MCPU.empty())
+        getDriver().Diag(diag::err_cpu_unsupported_isa) << CPU << "ARM";
+       else
+        getDriver().Diag(diag::err_arch_unsupported_isa)
+          << tools::arm::getARMArch(MArch, getTriple()) << "ARM";
+    }
+
     // Assembly files should start in ARM mode, unless arch is M-profile.
     // Windows is always thumb.
     if ((InputType != types::TY_PP_Asm && Args.hasFlag(options::OPT_mthumb,
diff --git a/test/Driver/arm-thumb-only-cores.c b/test/Driver/arm-thumb-only-cores.c
new file mode 100644
index 0000000000000000000000000000000000000000..cf40bf0ffa4a2ae1dce4cc325a7218861e7ebbcb
--- /dev/null
+++ b/test/Driver/arm-thumb-only-cores.c
@@ -0,0 +1,12 @@
+// RUN: not %clang -target arm-unknown-linux -marm -mcpu=cortex-m0 %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix M0 %s
+// M0: error: CPU 'cortex-m0' does not support 'ARM' execution mode
+
+// RUN: not %clang -target arm-unknown-linux -marm -march=armv7m %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix ARMV7M %s
+// RUN: not %clang -target armv7m-unknown-linux -mno-thumb %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix ARMV7M %s
+// ARMV7M: error: Architecture 'armv7m' does not support 'ARM' execution mode
+//
+// RUN: %clang -S -emit-llvm -target arm-unknown-linux -mcpu=cortex-m0 %s -o /dev/null 2>&1
+// M-Profile CPUs default to Thumb mode even if arm triples are provided.