Liberal Premier Brian Gallant has called a byelection for Nov. 17.
"We thought it was important for the people of Saint John East to be represented before we go into the legislature," Gallant said on Saturday.
"Although we don't have a date yet, we know it will be before Christmas, so we thought it was important to have the byelection as soon as possible."
The seat became vacant Tuesday after Liberal Gary Keating quit politics, saying the hours and travel that come with the job would have a negative impact on his health and family.
His sudden departure came about three weeks after the provincial election, and before he was even sworn-in as a member of the legislature.
Interim Progressive Conservative Leader Bruce Fitch blamed a lack of leadership by Gallant for Keating's departure.
"If he was a better leader, he could have kept their caucus together, and he could have prevented a byelection," Fitch said.
Gallant said the reason Keating quit has been given, and people shouldn't be looking for other reasons.
Gallant said that because of the tight timelines, the Liberal party has given him the power to appoint a candidate.
He said he'd like to see another woman in the Liberal caucus.
"But we'll try to pick somebody based on what they can do and deliver and how well they can represent the people of Saint John East," he said.
Jason Stephen, the president of New Brunswick's Progressive Conservative party, said Glen Savoie will get the nod as their candidate in Saint John East.
The former MLA lost to Keating in last month's election by just nine votes.
Members of the New Democratic party have launched an effort to convince their leader, Dominic Cardy, to run in the riding. The New Democrats have no seats in the legislature.
Keating's resignation leaves the Liberals with 26 seats.
The Progressive Conservatives have 21 members and the Greens have one.