Castlefin, sometimes spelt Castlefinn, is a market town and townland in the Finn Valley of County Donegal in Ulster, the northern province in Ireland. It is located between Ballybofey and Lifford in East Donegal and, as of 2016, the population was 705. The River Finn flows by the town. The town is located in along the main N15 national primary road, which runs from Bundoran to Lifford. The town lies 6 miles from Lifford and 8 miles from the twin towns of Ballybofey and Stranorlar. It has close links to Letterkenny, to the twin towns of Ballybofey and Stranorlar, and to West Tyrone in Northern Ireland, especially with the towns of Strabane and Castlederg.

The River Finn is a river in the west of Ulster, the northern province in Ireland. The river mainly flows through County Donegal, part of the Republic of Ireland, while a short stretch of the river also partially flows through County Tyrone, part of Northern Ireland. From Lough Finn, the river goes to Ballybofey and Stranorlar before joining the River Mourne at Lifford and Strabane.