Antrim and Newtownabbey Councillor John Blair is to succeed David Ford as Alliance MLA for South Antrim.
The 52-year-old, who the party says will become the first openly gay Assembly member, will resign his council seat before taking up his new role early next week.
The Glengormley Urban ward representative and former deputy mayor said: “I am honoured and delighted to have been selected as the new Alliance MLA for South Antrim by my colleagues and party members.
“I want to also pay tribute to David Ford, whose leadership and vision has helped grow the party both in the constituency and wider.”
Mr Ford, a former party leader and Justice Minister, announced earlier this week he is stepping down after 20 years as an MLA.
Mr Blair added: “I will be looking to build on my previous work as councillor and provide a strong Alliance voice for local people here in South Antrim and also in the Assembly. We are a growing party and we are already hitting the ground running ahead here of next year’s council elections.
“Since deciding to run for the MLA seat, I have received support and encouragement from right across the community, showing the progressive and positive alternative Alliance represents.
“I want to be carrying out all the duties of an MLA, so it is vital the Secretary of State calls all-party talks with an independent mediator immediately, so we can all get back to Stormont and do the job people want us to do.”