SINN Féin has selected Maolíosa McHugh to replace Michaela Boyle as West Tyrone MLA.
Ms Boyle won a seat on Derry City and Strabane District Council for the Sperrin area in last week's elections, leading to her giving up her Stormont position.
Mr McHugh, a former mayor for Derry, lost his seat on the council for the Derg area in the local government poll.
Sinn Fein West Tyrone MP Órfhlaith Begley welcomed the co-option announcement.
"Delighted that Maolíosa McHugh is our new MLA for West Tyrone. A true republican with a wealth of knowledge and experience," she said.
The MP also paid tribute to Ms Boyle as "one of the most hard-working elected representatives".
Northern Ireland has not had a functioning Stormont assembly since the DUP and Sinn Féin-led executive fell apart in 2017 in the wake of the RHI scandal.
Fresh talks have been convened between the main parties in a bid to restore the devolved institutions.
