Grimsthorpe Lake is a lake in the Moira River and Lake Ontario drainage basins that straddles the border between Tweed, Hastings County and Addington Highlands, Lennox and Addington County in Ontario, Canada. The lake is about 1.9 km long and 0.9 km wide and lies at an elevation of 319 m about 11 km east of the community of Gunter and 17 km northwest of the community of Cloyne. The primary inflow is Partridge Creek from the northwest, and there are three unnamed creek secondary inflows at the southwest, north and south. Partridge Creek is also the primary outflow at the southeast. It flows via the Skootamatta River and Moira River into the Bay of Quinte on Lake Ontario at Belleville.