The River Lagan (from Irish Abhainn an Lagáin 'river of the low-lying district'; Ulster Scots: Lagan Wattèr) is a major river in Northern Ireland which runs 53.5 mi (86.1 km) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea. The Lagan forms much of the border between County Antrim and County Down in the east of Ulster. It rises as a stream near to the summit of Slieve Croob. It runs to Belfast through Dromara, Donaghcloney and Dromore. On the lower slopes of the mountain, it combines with a branch from Legananny Mountain, just opposite Slieve Croob. The river then turns east to Magheralin into a broad plain between the plateaus of Antrim and Down.

The river drains approximately 609 square km of agricultural land and flows to the Stranmillis Weir, from which point on it is estuarine. The catchment consists mainly of enriched agricultural grassland in the upper parts, with the lower section draining urban Belfast and Lisburn. There is one significant tributary, the Ravernet River, and there are several minor tributaries, including the Carryduff River, the River Farset and the Blackstaff River. Work is proceeding to restore a self-sustaining population of Atlantic salmon to the river.

Ptolemy's Geography (2nd century AD) described a river called Λογια (Logia). The river name is thought to connect with Old Irish loeg ("calf") and with *laks ("salmon").

The name Belfast originates from the Irish Béal Feirste, or the mouth of the Farset, the river on which the city was built and which flows into the Lagan. The Farset has been superseded by the River Lagan as the most important river. A section of the Farset is covered over by the city's High Street.

In 1989 the Laganside Corporation was established by the British government to redevelop the areas surrounding the Lagan in Belfast. Major developments of the Laganside Corporation along the river include the regeneration of the city's former Gasworks, and the Lanyon Place development, which includes the Waterfront Hall.

One of the earliest and most important undertakings of the corporation was the Lagan Weir. Completed in 1994 at a cost of £14 million (equivalent to £27.02 million in 2023), the weir controls the level of water upstream. One of the main functions of the weir was to reduce unsightly mud flats at low tide. This was mostly successful, but mud flats are still evident on the river. The weir is a series of large steel barriers which are raised as the tide retreats to keep the river at an artificially constant level. Dredging and aeration have increased water quality in the river.

The river is used by a number of rowing clubs, including Queen's University Boat Club, Queen's Ladies Boat Club, Methodist College Boat Club, Royal Belfast Academical Institution (RBAI) Rowing Club, Belfast Rowing Club (BRC) and Lagan Scullers Club. The Boathouses are all based between the Governors Bridge and the Stranmillis Weir.

Lisburn City Council built the Lagan Valley Island complex; a new headquarters for the council and an Arts Centre, wedding and conference facilities and a restaurant. Opened in 2001, the building is surrounded by the Lagan on one side and a channel linked to the river on the other.^{[citation needed]}