diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -295,6 +295,14 @@
   add_definitions(-DFLANG_REPOSITORY_STRING="${FLANG_REPOSITORY_STRING}")
 endif()
 
+include(TestBigEndian)
+test_big_endian(IS_BIGENDIAN)
+if (IS_BIGENDIAN)
+  add_compile_definitions(FLANG_BIG_ENDIAN=1)
+else ()
+  add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
+endif ()
+
 # Configure Flang's Version.inc file.
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/include/flang/Version.inc.in
diff --git a/flang/include/flang/Evaluate/common.h b/flang/include/flang/Evaluate/common.h
--- a/flang/include/flang/Evaluate/common.h
+++ b/flang/include/flang/Evaluate/common.h
@@ -133,9 +133,9 @@
 
 static constexpr Rounding defaultRounding;
 
-#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#if FLANG_BIG_ENDIAN
 constexpr bool isHostLittleEndian{false};
-#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#elif FLANG_LITTLE_ENDIAN
 constexpr bool isHostLittleEndian{true};
 #else
 #error host endianness is not known
diff --git a/flang/runtime/environment.h b/flang/runtime/environment.h
--- a/flang/runtime/environment.h
+++ b/flang/runtime/environment.h
@@ -14,9 +14,9 @@
 
 namespace Fortran::runtime {
 
-#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#if FLANG_BIG_ENDIAN
 constexpr bool isHostLittleEndian{false};
-#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#elif FLANG_LITTLE_ENDIAN
 constexpr bool isHostLittleEndian{true};
 #else
 #error host endianness is not known