Loch Long (Gaelic for Ship Lake, Long being the word for ship) is a body of water in Argyll and Bute, Scotland. The sea loch extends from the Firth of Clyde at its southwestern end. It measures approximately 20 miles in length, with a width of between one and two miles. The loch also has an arm, Loch Goil, on its western side. Loch Long was historically the boundary between Argyll and Dunbartonshire. However in 1996 boundary redrawing meant that it moved wholly within the council area of Argyll and Bute.
The loch was used as a testing ground for torpedoes during World War II and contains numerous wrecks. It is now a popular area for sport diving.The Ardentinny Outdoor Education Centre (on the other bank) also uses the loch for watersports.
The Finnart Oil Terminal is located on the eastern shore of the loch, linked to the Grangemouth Refinery via a 58 miles (93 km) long pipeline. The eastern shore is also the location of the Royal Navy's Coulport Armament depot, part of HMNB Clyde, and the Glen Mallan jetty, linked to Glen Douglas defence munitions depot.
Important villages on the loch include Arrochar at its head and Cove on the east shore near its foot.
The loch forms the entire western coastline of the Rosneath Peninsula.