diff --git a/src/doomdef.h b/src/doomdef.h
index 88575ea7c..e6327998a 100644
--- a/src/doomdef.h
+++ b/src/doomdef.h
@@ -101,6 +101,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#define _USE_MATH_DEFINES // fixes M_PI errors in r_plane.c for Visual Studio
 #include <math.h>
 
 #ifdef GETTEXT