diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1270,7 +1270,7 @@
            "of 0 means the number of threads will be derived from "
            "the number of CPUs detected)">;
 def fthinlto_index_EQ : Joined<["-"], "fthinlto-index=">,
-  Flags<[CC1Option]>, Group<f_Group>,
+  Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Perform ThinLTO importing using provided function summary index">;
 def fmacro_backtrace_limit_EQ : Joined<["-"], "fmacro-backtrace-limit=">,
                                 Group<f_Group>, Flags<[DriverOption, CoreOption]>;
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
@@ -2119,6 +2119,13 @@
               Diag(clang::diag::warn_drv_treating_input_as_cxx)
                   << getTypeName(OldTy) << getTypeName(Ty);
           }
+
+          // If the driver is invoked as clang-cl with -thinlto-index=,
+          // extensions that normally identify native object files actually
+          // identify LLVM bitcode files.
+          if (IsCLMode() && Ty == types::TY_Object &&
+              Args.hasArgNoClaim(options::OPT_fthinlto_index_EQ))
+            Ty = types::TY_LLVM_BC;
         }
 
         // -ObjC and -ObjC++ override the default language, but only for "source
diff --git a/clang/test/Driver/cl-thinlto-backend.c b/clang/test/Driver/cl-thinlto-backend.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/cl-thinlto-backend.c
@@ -0,0 +1,9 @@
+// RUN: %clang_cl -c -flto=thin -Fo%t.obj %s
+// RUN: llvm-lto2 run -thinlto-distributed-indexes -o %t.exe %t.obj
+
+// -fthinlto_index should be passed to cc1
+// RUN: %clang_cl -### -c -fthinlto-index=%t.thinlto.bc -Fo%t1.obj \
+// RUN:     %t.obj 2>&1 | FileCheck %s
+
+// CHECK: -fthinlto-index=
+// CHECK: "-x" "ir"