"In order for our team to reach its full potential, we determined a coaching change was necessary. Our team is capable of more than we have demonstrated this season," general manager Kelly McCrimmon said in a statement.
The team has named Peter DeBoer its new head coach, and he will join the team Thursday.
Gallant had been the Golden Knights' only coach since the franchise's inception three years ago and led the squad to a surprising trip to the Stanley Cup Final in the inaugural 2017-18 season.
The squad reached the playoffs again in 2018-19 after finishing third in the Pacific Division but suffered a first-round loss, while the team is now in a fight to just reach the postseason.
Vegas is tied for the second and last wild-card spot in the Western Conference with 54 points, but it has played two more games than division rival Vancouver Canucks and the Central Division's Winnipeg Jets. Four losses in a row also shows the team isn't trending in the right direction heading into the All-Star Weekend, which is Jan. 24-26.
Though he won the Jack Adams Award for Coach of the Year in 2017-18, Gallant hasn't made it past three seasons at any of his three coaching stops (Columbus and Florida).
The Golden Knights will now try to turn around the season under DeBoer, who was fired as head coach of the San Jose Sharks in December.
The 51-year-old has spent parts of eight seasons as an NHL head coach, leading the Sharks, New Jersey Devils and Florida Panthers. He owns a 415-329-111 career record and has led two different teams to the Stanley Cup Final.
"In Peter DeBoer, we have a proven, experienced head coach who we believe can help us achieve our ultimate goal," McCrimmon said.
Despite this being a midseason hire, DeBoer is expected to remain at the helm beyond this season.