Lagan Valley is a Northern Ireland parliamentary constituency in the House of Commons of the United Kingdom covering the area of the Lagan Valley including the south part of County Antrim and the north-west part of County Down.
Its MP since the 2024 general election is Sorcha Eastwood of the Alliance Party. This was the first time since its creation it did not elect a unionist MP.
The seat was created in 1983, as part of an expansion of Northern Ireland's constituencies from 12 to 17, and was predominantly made up from parts of South Antrim and North Down in the area of the River Lagan. In their original proposals, in January 1980, the boundary commission proposed calling it "Lagan". In further revisions in 1995 it lost some areas to both Belfast West and Strangford.
MPs from Lagan Valley have included two party leaders: James Molyneaux, leader of the Ulster Unionist Party from 1979 to 1995 (having represented South Antrim from 1970 to 1979), and Jeffrey Donaldson, leader of the Democratic Unionist Party from 2021 to 2024, when he resigned as party leader and was suspended as a party member.
This seat saw a swing towards the Alliance Party of over 17%, and the largest decrease in vote share for the DUP at the 2019 general election.