Sinn Féin's Martina Anderson has been selected to replace Raymond McCartney as an MLA for Foyle.
It marks a return to the Assembly for the outgoing MEP, who served as an MLA for the constituency between 2007 and 2012.
Ms Anderson's appointment was confirmed on Saturday at a party convention in Londonderry.
Speaking afterwards, Martina Anderson said: "I am proud and honoured to have been selected at today's selection convention in Derry to be co-opted onto the Assembly as an MLA for Foyle.
"I want to pay tribute to Raymond McCartney for the first class leadership he has shown in this city for many years. His dedication to serving the people of Foyle has always been an inspiration.”
Ms Anderson's role as an MEP ended on Friday night when the United Kingdom officially left the EU.
Commenting on the timing of her new appointment Ms Anderson said: "I have been chosen to join the Assembly at a time of great challenge.”
All three Northern Ireland MEPs who were elected to the European Parliament last May, will now sit in the Assembly chamber. Alliance's Naomi Long and DUP veteran Diane Dodds both now hold Executive roles.
Raymond McCartney, who served as an MLA for Foyle for 15 years, announced that he was stepping down on Thursday.