Veteran Antrim councillor Adrian Cochrane-Watson has been chosen to succeed Danny Kinahan as South Antrim MLA.
Mr Kinahan was elected as MP for the area in May and said he would relinquish his Assembly role as soon as possible.
The Ulster Unionist Party met last night (Tuesday) with Mr Cochrane-Watson being chosen to replace Mr Kinahan in South Antrim seat.
“I am personally delighted to be selected to carry on the excellent work of Danny Kinahan MP,” Mr Watson said.
He added: “I will represent the entire community of South Antrim in the Assembly.
“I would like to thank my friends and colleagues for their continued support.”
UUP leader Mike Nesbitt said: “This is a moment of history for the Ulster Unionist Party, the first time since we secured devolution in 1998 that we have had to replace MLAs because they have gone on to become Members of Parliament.
“I can think of no better sign that the Party is in growth mode.
“We are losing two excellent MLAs in Danny Kinahan and Tom Elliott and on behalf of the Ulster Unionist Party, I thank them for all they have done for the Party grouping at Stormont and more importantly for their constituents, whom they will continue to serve in their new positions at Westminster.
“I rest assured they are being replaced by two equally excellent choices. Adrian and Neil will bring energy, passion and commitment to their new roles and I know they will work hard in association with Danny and Tom for the people of South Antrim and Fermanagh South Tyrone respectively.
“Adrian and Neil had to win through against stiff opposition, another sure sign that the Ulster Unionist Party is in good heart. The public recognise we have three key political attributes: momentum, belief and credibility.
“I look forward to working with Adrian and Neil to further enhance the voters’ confidence that what is currently on offer from the two parties at the heart of government is not as good as it gets, and that the Ulster Unionist Party has the people and the policies to offer better.”