Sully Island (Welsh: Ynys Sili) is a small tidal island and Site of Special Scientific Interest at the hamlet of Swanbridge, Vale of Glamorgan, four hundred and fifty metres off the northern coast of the Bristol Channel, midway between the towns of Penarth and Barry and 7 miles (11 kilometres) south of the Welsh capital city of Cardiff. Access to the island is on foot at low tide from the car park of the Captain's Wife public house. It is 14 and a half acres in extent.