diff --git a/os/macosx/macos.h b/os/macosx/macos.h
index c01bfe84c7..491f4ff090 100644
--- a/os/macosx/macos.h
+++ b/os/macosx/macos.h
@@ -2,5 +2,18 @@
 #define MACOS_H
 
 void ShowMacDialog ( const char *title, const char *message, const char *buttonLabel );
+void ShowMacAssertDialog ( const char *function, const char *file, const int line, const char *expression );
 
-#endif /* MACOS_H */
\ No newline at end of file
+// Since MacOS X users will never see an assert unless they started the game from a terminal
+// we're using a custom assert(e) macro.
+#undef assert
+
+#ifdef NDEBUG
+#define assert(e)       ((void)0)
+#else
+
+#define assert(e) \
+		(__builtin_expect(!(e), 0) ? ShowMacAssertDialog ( __func__, __FILE__, __LINE__, #e ): (void)0 )
+#endif
+
+#endif /* MACOS_H */
diff --git a/os/macosx/macos.m b/os/macosx/macos.m
index f09a8d3d4d..936aaa7a02 100644
--- a/os/macosx/macos.m
+++ b/os/macosx/macos.m
@@ -13,3 +13,14 @@ void ShowMacDialog ( const char *title, const char *message, const char *buttonL
 	NSRunAlertPanel([NSString stringWithCString: title], [NSString stringWithCString: message], [NSString stringWithCString: buttonLabel], nil, nil);
 }
 
+void ShowMacAssertDialog ( const char *function, const char *file, const int line, const char *expression )
+{
+	const char *buffer = 
+			[[NSString stringWithFormat:@"An assertion has failed and OpenTTD must quit.\n%s in %s (line %d)\n\"%s\"\n\nYou should report this error the OpenTTD developers if you think you found a bug.",
+			function, file, line, expression] cStringUsingEncoding:NSASCIIStringEncoding];
+	NSLog(@"%s", buffer);
+	ShowMacDialog( "Assertion Failed", buffer, "Quit" );
+	
+	// abort so that a debugger has a chance to notice
+	abort();
+}
diff --git a/stdafx.h b/stdafx.h
index f78bf998c7..da8f1e88ba 100644
--- a/stdafx.h
+++ b/stdafx.h
@@ -20,10 +20,16 @@
 #endif
 
 #include <stdio.h>
-#include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 
+// MacOS X will use an NSAlert to display failed assertaions since they're lost unless running from a terminal
+#if !defined(__APPLE__)
+#include <assert.h>
+#else
+#include "os/macosx/macos.h"
+#endif
+
 #if defined(UNIX) || defined(__MINGW32__)
 #	include <sys/types.h>
 #endif
diff --git a/unix.c b/unix.c
index 24f3ce5c1d..414e0baf24 100644
--- a/unix.c
+++ b/unix.c
@@ -43,7 +43,6 @@ ULONG __stack = (1024*1024)*2; // maybe not that much is needed actually ;)
 		//the mac implementation needs this file included in the same file as main()
 		#include <SDL.h>
 	#endif
-	#include "os/macosx/macos.h"
 #endif
 
 static char *_fios_path;