Lough Foyle, sometimes Loch Foyle, is the estuary of the River Foyle, on the north coast of Ireland. It lies between County Londonderry in Northern Ireland and County Donegal in the Republic of Ireland. Sovereignty over the waters has been in dispute since the Partition of Ireland.

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.