Emergency services were called out to the scene of the incident near Lochans at about 11:40.
Police said that the road was likely to be closed for a significant amount of time as a result of the crash.
Drivers have been advised to avoid the area if possible or expect delays. Diversions are being put in place.
The A77 has been shut south of Stranraer following a serious accident involving a motorbike and a car.