Alasdair McDonnell has said he will give up his 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 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.