File:Banks Of The River Lagan - Near Queens Bridge In Belfast (5688382886).jpg

The River Lagan is a major river in Northern Ireland which runs 40 miles (60 km) from the Slieve Croob mountain in County Down to Belfast where it enters Belfast Lough, an inlet of the Irish Sea. The River Lagan forms much of the border between County Antrim and County Down. It rises as a tiny fast moving stream off the Transmitter road near to the summit of Slieve Croob. From here it continues on its journey to Belfast through Dromara and Dromore. On the lower slopes of the mountain it is joined by another branch from Legananny (Cratlieve) Mountain, just opposite Slieve Croob. At Dromara, about four miles from its source, its height above the sea is 390 ft (119m). As the river continues on its journey to Belfast it turns east to Magheralin into a broad plain between the Antrim plateau and the plateau of Down.

The river drains approximately 609 square km of agricultural land and flows over 70 km from the Mourne Mountains to the Stranmillis Weir, from which point on it is estuarine. The catchment consists mainly of enriched agricultural grassland in the upper parts, with a 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. Water quality is generally fair though there are localised problems and occasional pollution incidents, mainly due to effluents from farms.

In 1989 the Laganside Corporation was established by the 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, the Odyssey entertainment and leisure development and the Lanyon Place development which includes the Waterfront Hall, in many ways the flagship of the corporation.