--- a/mesonbuild/utils/universal.py
+++ b/mesonbuild/utils/universal.py
@@ -1019,6 +1019,8 @@
 
 
 def default_prefix() -> str:
+    if is_windows() and 'MSYSTEM' in os.environ:
+        return sys.prefix
     if is_windows():
         return 'c:/'
     if is_haiku():
