St Donat's Castle is a medieval castle in the Vale of Glamorgan, Wales, overlooking the Bristol Channel in the village of St Donat's near Llantwit Major, and about 25 km west of Cardiff. Since 1962 the castle has housed the international Sixth form college Atlantic College. The castle lies on an elevation to the east of a small valley opening to the sea between precipitous sea cliffs to both east and west.