Lough Swilly (Loch Súilí in Irish) in Ireland is a fjord-like body of water lying between the eastern side of the Inishowen Peninsula in County Donegal and the rest of northern Donegal.

At the northern extremeties of the lough are Inishowen Head and Dunaff Head. Towns situated on the Lough include Buncrana on Inishowen and Rathmullan on the western side. At the southern end of the Lough lies Letterkenny.

After a failed general uprising, in September 1607, Hugh O'Neill, 2nd Earl of Tyrone and Rory O'Donnell, 1st Earl of Tyrconnell set sail from Rathmullan with ninety of their followers in the Flight of the Earls never to see Ireland again.

The Lough was used as an anchorage by the Royal Navy during World War I. The British also built a number of forts to protect the Lough and the remains of these can be seen at Lenan Head, Dunree (now a museum) and at Buncrana. The Lough was also one of the Treaty Ports specified in the Anglo-Irish Treaty.

See also: List of Irish lochs and loughs