The Firth of Forth is the estuary or firth of Scotland's River Forth, where it flows into the North Sea, between Fife to the north, and West Lothian, the City of Edinburgh and East Lothian to the south. It was known as Bodotria in Roman times.

Geologically, the Firth of Forth is a fjord, formed by the Forth Glacier in the last glacial period.

There are many towns which line the shores, as well as the petrochemical complexes at Grangemouth, the commercial docks at Leith, former oilrig construction yards at Methil, the ship-breaking facility at Inverkeithing and the naval dockyard at Rosyth, with numerous other industrial areas including the Forth Bridgehead area (i.e., Rosyth, Inverkeithing and the southern edge of Dunfermline), Burntisland, Kirkcaldy, Bo'ness and Leven.

The Kincardine Bridge and the Forth Road Bridge and Forth Bridge carry traffic across the firth. A fourth crossing, located next to the Kincardine Bridge, opened in 2008. On 1 October 2008 it was announced that the new bridge would be called the Clackmannanshire Bridge. Queensferry Crossing, another new road bridge is under construction alongside the Forth Road Bridge and is expected to be open in 2016.

In July 2007, a hovercraft passenger service completed a two-week trial between Portobello, Edinburgh and Kirkcaldy, Fife. The trial of the service (marketed as "Forthfast") was hailed as a major operational success, with an average passenger load of 85%. It was estimated the service could cut congestion for commuters on the Forth road and rail bridges by carrying about 870,000 passengers a year. The plans were shelved in December 2011.