Harry Avery's Castle is situated half a mile south-west of Newtownstewart, in County Tyrone, Northern Ireland. It is a rare example of a stone castle built by a native Gaelic Irish chief, although its origins and history are uncertain. It is associated with Henry Aimbreidh O'Neill (died 1392), anglicised as Harry Avery, and is certainly named after him. It is in the townland of Upper or New Deer Park, in the Derry and Strabane District Council area, at grid reference H3915 8527. The remains of the castle are a State Care Monument under the guardianship of the Northern Ireland Environment Agency.