Lough Foyle (Loch Feabhail in Irish) is the name given to the estuary of the River Foyle. It starts where the Foyle leaves Londonderry. It separates the Malin peninsula from Northern Ireland.

In the summer time, a ferry service operates between County Donegal and County Londonderry over Lough Foyle.

There is a saying, that when one is on a boat on Lough Foyle, one is at the only place in the world where north is south, and south is north. (The Republic of Ireland, even Donegal, is referred to as "The South" in Northern Ireland, while "The North" is a common term for Northern Ireland).

See also: List of Irish lochs and loughs