Firth is the Scots word used to denote a long narrow arm of the sea in Scotland, which may be part of an estuary, or just an inlet, or even a strait (as in the case of the Pentland Firth).

A firth is very often associated with a large river, where erosion caused by the tidal effects of incoming sea water passing upriver has widened the riverbed to an estuary, such as may be seen in the Firth of Clyde. However, this cannot be said in every case. The Cromarty Firth on the East coast of Scotland, for example, resembles a large loch with only a relatively small outlet to the sea.

See Pentland Firth.

The Firths on the West Coast of Scotland from North to South

The Firths on the East coast of Scotland from North to South (these flow into the North Sea)

See also: List of waterways