The River Foyle is a river in west Ulster in the northwest of the island of Ireland, which flows from the confluence of the rivers Finn and Mourne at the towns of Lifford in County Donegal, Republic of Ireland, and Strabane in County Tyrone, Northern Ireland. From here it flows to the city of Derry, where it discharges into Lough Foyle and, ultimately, the Atlantic Ocean. The total length of the River Foyle is 32 km (20 mi). The river separates part of County Donegal from parts of both County Londonderry and County Tyrone. The district of County Donegal that borders the western bank of the River Foyle is traditionally known as the Laggan. This district includes the villages of St Johnston and Carrigans, both of which are nestled on the banks of the River.

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.