diff --git a/src/cstdio.cpp b/src/cstdio.cpp
index 0aa46ed7..2ff9574c 100644
--- a/src/cstdio.cpp
+++ b/src/cstdio.cpp
@@ -8,7 +8,9 @@
 #define BOOST_NOWIDE_SOURCE
 
 #ifdef _MSC_VER
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
+#endif
 #elif defined(__MINGW32__) && defined(__STRICT_ANSI__)
 // Need the _w* functions which are extensions on MinGW but not on MinGW-w64
 #include <_mingw.h>
diff --git a/src/cstdlib.cpp b/src/cstdlib.cpp
index 4234a715..ab0d4455 100644
--- a/src/cstdlib.cpp
+++ b/src/cstdlib.cpp
@@ -8,7 +8,9 @@
 #define BOOST_NOWIDE_SOURCE
 
 #ifdef _MSC_VER
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
+#endif
 #elif defined(__MINGW32__) && defined(__STRICT_ANSI__)
 // Need the _w* functions which are extensions on MinGW but not on MinGW-w64
 #include <_mingw.h>
diff --git a/test/test_system.cpp b/test/test_system.cpp
index 2c0e37e7..76171375 100644
--- a/test/test_system.cpp
+++ b/test/test_system.cpp
@@ -5,8 +5,10 @@
 // https://www.boost.org/LICENSE_1_0.txt
 
 #ifdef _MSC_VER
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
 #endif
+#endif
 
 #include <boost/nowide/args.hpp>
 #include <boost/nowide/cstdlib.hpp>