Larne councillor Gordon Lyons is to replace the DUP's Sammy Wilson in the assembly.
The East Antrim MP announced he was leaving Stormont earlier this month to concentrate on his Westminster job.
New rules outlawing double-jobbing will come into force in March next year.
"It is a huge honour and a privilege to have been selected to replace Sammy as an MLA for East Antrim," Mr Lyons said.
Mr Wilson said: "I have worked closely with Gordon over a number of years and know how dedicated he is to the constituency."
