Southwest (Icelandic: Suðvestur) is one of the six multi-member constituencies of the Althing, the national legislature of Iceland. The constituency was established as Reykjanes in 1959 following the nationwide extension of proportional representation for elections to the Althing. It was renamed Southwest in 2003 when the Reykjanes area of constituency was merged into the Southern constituency following the re-organisation of constituencies across Iceland. Southwest is conterminous with the Capital region but excludes Reykjavík Municipality which has its own constituencies. The constituency currently elects 11 of the 63 members of the Althing using the open party-list proportional representation electoral system. At the 2021 parliamentary election it had 73,699 registered electors.

Southwest currently elects 11 of the 63 members of the Althing using the open party-list proportional representation electoral system.^[4][5] Constituency seats are allocated using the D'Hondt method.^[6][7] Compensatory seats (equalisation seas) are calculated based on the national vote and are allocated using the D'Hondt method at the constituency level.^[8][9] Only parties that reach the 5% national threshold compete for compensatory seats.^[7][10]