Democratic Unionist MEP Diane Dodds is to replace Carla Lockhart as assembly member for Upper Bann.
Ms Lockhart MP resigned from the Northern Ireland Assembly after she became MP for the constituency in this month's general election.
Mrs Dodds has been an MEP since 2009, but will lose that post after the UK leaves the European Union.
"Carla has been a first class advocate for the constituency," Mrs Dodds said.
"I will be working with the DUP team in Upper Bann to build on her reputation for delivery."
DUP leader Arlene Foster said: "Diane will be a first class advocate for the people of Upper Bann and brings a wealth of experience to the post.
"Having served in council, the assembly and then for two terms as an MEP, Diane has a proven record of delivering for the people she represents."