Alasdair McDonnell has said he will give up his Northern Assembly seat to concentrate on remaining an MP.
The SDLP leader retained his South Belfast Westminster position with 9,560 votes. He was first elected there in 2005.
Mr McDonnell told the BBC’s Sunday Politics Show he would “set processes in train” to find a Stormont replacement.
He said: “I will be talking to the party about the process of moving now that I have secured the Westminster seat for the third time and that’s a major achievement.
“South Belfast is still a unionist majority constituency.”
The SDLP also held on to Westminster seats in Foyle and South Down.
Last year the Northern Ireland Office passed an act banning MPs from double-jobbing.
The law comes into force when the Assembly is dissolved, expected next year for fresh elections.
The DUP has retained its position as the largest party in the North but the UUP is arguably celebrating harder after landing two surprise wins in the General Election.
The Ulster Unionists are proclaiming a renaissance after candidates Danny Kinahan and Tom Elliott won in South Antrim and Fermanagh and South Tyrone respectively to herald the party’s return to parliament after a period without any representation.
Mr Elliott won Fermanagh and South Tyrone from Sinn Fein’s Michelle Gildernew - a rare loss of a Westminster seat for the republican party.
While Mr Kinahan ousted DUP veteran Willie McCrea in South Antrim, the DUP found consolation with a much-craved triumph in East Belfast, where former Belfast city mayor Gavin Robinson recaptured the seat his party lost to the Alliance Party’s Naomi Long in 2010.