DUP councillor Paul Rankin has been co-opted to replace Edwin Poots as an MLA for the party in Lagan Valley.
He fills the vacancy that arose in the constituency after the Agriculture Minister switched to South Belfast following the sudden death of Christopher Stalford.
Mr Rankin will be in the role for just two weeks ahead of the dissolution of the Stormont Assembly at the end of March for fresh elections.
Read more: Christopher Stalford to be replaced by Edwin Poots in South Belfast, DUP leader announces
The Electoral Office for Northern Ireland confirmed the co-option on Monday.
The DUP has not yet confirmed which candidates it will run in the Lagan Valley constituency.
DUP leader Sir Jeffrey Donaldson, who represents Lagan Valley at Westminster, has previously said he intends to run as a candidate in the constituency for the Assembly election in May.
He said in August: "I intend to stand in the Assembly elections if I am unable to return to the Assembly before that."
However, a return to Stormont would mean Sir Jeffrey would have to give up his MP seat, triggering a by-election.
On Friday, Sir Jeffrey said that in Lagan Valley his party was reviewing whether it will run three candidates, as it intended, to maximise unionist representation.
Former First Minister Paul Givan and Mr Rankin are now the DUP's two MLAs for Lagan Valley.
In a statement Sir Jeffrey welcomed the co-option of Mr Rankin.
He said: "Paul has a sterling record of hard work and delivery for the people of this area, with over 20 years of experience as an elected representative.
"I have worked closely with Paul for many years to champion the case for Lagan Valley and its people. I know he will be a diligent and dedicated servant of their interests in the Northern Ireland Assembly."
Read more: Christopher Stalford to be replaced by Edwin Poots in South Belfast, DUP leader announces
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