diff --git a/clang/lib/Basic/Targets/OSTargets.h b/clang/lib/Basic/Targets/OSTargets.h
--- a/clang/lib/Basic/Targets/OSTargets.h
+++ b/clang/lib/Basic/Targets/OSTargets.h
@@ -673,6 +673,7 @@
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("_IBMR2");
     Builder.defineMacro("_POWER");
+    Builder.defineMacro("__THW_BIG_ENDIAN__");
 
     Builder.defineMacro("_AIX");
     Builder.defineMacro("__TOS_AIX__");
diff --git a/clang/test/Preprocessor/init-ppc.c b/clang/test/Preprocessor/init-ppc.c
--- a/clang/test/Preprocessor/init-ppc.c
+++ b/clang/test/Preprocessor/init-ppc.c
@@ -538,6 +538,7 @@
 // PPC-AIX:#define __SIZE_MAX__ 4294967295UL
 // PPC-AIX:#define __SIZE_TYPE__ long unsigned int
 // PPC-AIX:#define __SIZE_WIDTH__ 32
+// PPC-AIX:#define __THW_BIG_ENDIAN__ 1
 // PPC-AIX:#define __TOS_AIX__ 1
 // PPC-AIX:#define __UINT16_C_SUFFIX__
 // PPC-AIX:#define __UINT16_MAX__ 65535