Liberal Rex Hillier took a narrow victory in the district of Conception Bay South, west of St. John's, on Wednesday evening, defeating Tory candidate Barry Petten.
Unofficial results from Elections Newfoundland and Labrador had Hillier with 2102 votes and Petten at 2026 after all 44 polls were counted.
The NDP's Cameron Mercer-Maillet ran a distant third, with just 130 votes.
The district is next door to the riding held by Paul Davis, who was sworn in as premier on Sept. 26.
The Tory government has now lost five straight byelections to the Opposition Liberals in the last 16 months, four of them in districts held by former cabinet ministers.
With the Liberal win, there are 29 Progressive Conservative members, 14 Liberals, three NDP and two vacant seats.
Political scientists have said Conception Bay South was a must-win for a government that has seen a series of cabinet ministers depart politics over the last year amid leadership struggles and a slide in the polls.
The riding was held by former justice minister Terry French, who retired from politics in September.
He won the district in a byelection in 2002. He succeeded his father, Bob French, who represented the Tories from 1996 until his death in 2002.
The Tory loss in Conception Bay South sets the stage for two more byelections later this month.
Voters will cast ballots Nov. 25 in the Humber East and Trinity-Bay de Verde districts.
Humber East and Trinity-Bay de Verde were held by former premier Tom Marshall and former finance minister Charlene Johnson, respectively.
Marshall retired from politics while Johnson resigned to be with her husband who works overseas.