Map - Firth of Clyde (Firth of Clyde)

The Firth of Clyde is the mouth of the River Clyde. It is located on the west coast of Scotland and constitutes the deepest coastal waters in the British Isles (it is 164 metres deep at its deepest). The firth is sheltered from the Atlantic Ocean by the Kintyre peninsula, which encloses the outer firth in Argyll and Ayrshire. The Kilbrannan Sound is a large arm of the Firth of Clyde, separating the Kintyre Peninsula from the Isle of Arran. Within the Firth of Clyde is another major island – the Isle of Bute. Given its strategic location at the entrance to the middle and upper Clyde, Bute played a vital naval military role during World War II.

At its entrance, the firth is about 26 mi wide. At one area in its upper reaches, it is joined by Loch Long and the Gare Loch. This area includes the large anchorage off of Greenock that is known as the Tail of the Bank. (The “Bank” is a reference to the sandbank and shoal that separates the firth from the estuary of the River Clyde.) Even where the sandbank narrows it, the Clyde is almost 2 mi wide. Its upper tidal limit is at the tidal weir adjacent to Glasgow Green.

The geographical (and popular) distinction between the firth and the River Clyde is vague. Some refer to Dumbarton as being “on the Firth of Clyde”. At the same time, the residents of Port Glasgow and Greenock often refer to the part of the firth that lies to the north of those areas as "the river". In Scottish Gaelic the landward end of the firth is called Linne Chluaidh (meaning the same as the English), while the area around the south of Arran, Kintyre and Ayrshire/Galloway is An Linne Ghlas.

The Firth encompasses many islands and peninsulas, and 12 ferry routes connect them to the mainland and to each other. The Firth is sometimes called the Clyde Waters, and is customarily considered to be part of the Irish Sea. The ferry services are run by Caledonian MacBrayne and by Western Ferries, and many of the routes are lifeline services for communities living in remote areas. A large number of sea lochs adjoin the Firth—-the largest being Loch Fyne.