diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6fb6362a..9efaa709 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -284,7 +284,7 @@ endif()
 if(FMILIB_BUILD_SHARED_LIB)
 	add_library(fmilib_shared SHARED ${FMILIB_SHARED_SRC})
 
-    if (UNIX)
+    if (UNIX AND NOT APPLE)
         target_compile_definitions(fmilib_shared PRIVATE -D_GNU_SOURCE)
     endif()
 
diff --git a/Config.cmake/jmutil.cmake b/Config.cmake/jmutil.cmake
index 0cce2f7c..1d2a8bd2 100644
--- a/Config.cmake/jmutil.cmake
+++ b/Config.cmake/jmutil.cmake
@@ -62,8 +62,11 @@ endif(CYGWIN)
 
 target_link_libraries(jmutils c99snprintf)
 
-if(UNIX)
+if(UNIX AND NOT APPLE)
     target_compile_definitions(jmutils PRIVATE -D_GNU_SOURCE)
+endif()
+
+if(UNIX)
 	target_link_libraries(jmutils dl)
 endif(UNIX)
 if(WIN32)
diff --git a/src/Util/src/JM/jm_portability.c b/src/Util/src/JM/jm_portability.c
index 093d6959..7608b5a6 100644
--- a/src/Util/src/JM/jm_portability.c
+++ b/src/Util/src/JM/jm_portability.c
@@ -15,8 +15,18 @@
 
 #include <stdlib.h>
 #include <stdio.h>
-#include <locale.h>
 #include <string.h>
+
+#include <locale.h>
+
+#if defined(_GNU_SOURCE) || defined(__APPLE__)
+#define UNIX_THREAD_LOCALE
+#endif
+
+#ifdef __APPLE__
+/* Include thread-specific locale functions for OSX. */
+#include <xlocale.h>
+#endif
 
 #include <fmilib_config.h>
 
@@ -357,7 +367,7 @@ int jm_snprintf(char * str, size_t size, const char * fmt, ...) {
 }
 
 struct jm_locale_t {
-#ifdef _GNU_SOURCE
+#ifdef UNIX_THREAD_LOCALE
     locale_t locale_old;
 #else
     char* locale_old;
@@ -375,7 +385,7 @@ jm_locale_t* jm_setlocale_numeric(jm_callbacks* cb, const char* value) {
 		return NULL;
 	}
 
-#ifdef _GNU_SOURCE
+#ifdef UNIX_THREAD_LOCALE
 	{
 		locale_t nloc = NULL;
 
@@ -446,7 +456,7 @@ int jm_resetlocale_numeric(jm_callbacks* cb, jm_locale_t* jmloc) {
 		return 1; /* impl. error */
 	}
 
-#ifdef _GNU_SOURCE
+#ifdef UNIX_THREAD_LOCALE
 	{
 		/* Get current locale, which is expected to have been set with a previous
 		 * call to 'jm_setlocale_numeric'. */