54.506°N 6.044°W / 54.506; -6.044

Lagan Valley was a single-member county constituency of the Parliament of Northern Ireland.

Before 1969, the area formed part of the Northern Ireland Parliament constituencies of Mid Down and Iveagh.

Lagan Valley was created by the Electoral Law Act (Northern Ireland) 1968 as a division of County Down. It was located to the south of Belfast. The seat included Hillsborough, Drumbo and Carryduff as well as the sprawling rural area of Ballymacbrennan, which are today split between the Lagan Valley and South Belfast constituencies.

The constituency sent one MP to the House of Commons of Northern Ireland at the 1969 Northern Ireland general election. The Parliament was prorogued on 30 March 1972, under the terms of the Northern Ireland (Temporary Provisions) Act 1972. It was formally abolished in 1973 when the Northern Ireland Constitution Act 1973 received Royal Assent on 18 July 1973.

The Parliamentary representative of the division was elected using the first-past-the-post system.

Porter joined the Alliance Party in June 1972, the Parliament having been suspended in March. As a result, he never sat as an Alliance member in the Northern Ireland House of Commons.

• Parliament prorogued 30 March 1972 and abolished 18 July 1973

• Northern Ireland Parliamentary Election Results 1921-1972, compiled and edited by Sydney Elliott (Political Reference Publications 1973)
• Northern Ireland House of Commons, 1921 - 1972

• For more information about the Northern Ireland House of Commons, see http://www.election.demon.co.uk/stormont/stormont.doc Archived 2006-02-11 at the Wayback Machine