diff --git a/src/portal.cpp b/src/portal.cpp
index 964f742c7..af98718bf 100644
--- a/src/portal.cpp
+++ b/src/portal.cpp
@@ -620,7 +620,7 @@ void P_TranslatePortalVXVY(line_t* src, double &velx, double &vely)
 	double orig_velx = velx;
 	double orig_vely = vely;
 	velx = orig_velx * port->mCosRot - orig_vely * port->mSinRot;
-	vely = orig_vely * port->mCosRot - orig_velx * port->mSinRot;
+	vely = orig_vely * port->mCosRot + orig_velx * port->mSinRot;
 }
 
 //============================================================================