Gallant, the first coach in Golden Knights history, was hired April 13, 2017, went 118-75-20 in three seasons with Vegas, and made the Stanley Cup Playoffs in each of the past two. Vegas advanced to the Stanley Cup Final in 2017-18, its first NHL season, and lost in five games to the Washington Capitals.
The Golden Knights (24-19-6), fifth in the Pacific Division and tied with the Winnipeg Jets for the second wild card into the playoffs from the Western Conference, also fired assistant Mike Kelly.
"With respect to what was behind the decision, as a manager, sometimes you have a feeling that something isn't the way you need it to be or want it to be," Golden Knights general manager Kelly McCrimmon said. "We feel we've underperformed a little bit, and certainly that's not to pile that at the feet of Mike and Gerard, but sometimes you feel a change is needed. So that was what went into the decision. Like anything we do, we try to do what we genuinely believe is in the best interest of the Golden Knights organization, and that's what we've done in this case."
Vegas has lost four straight games following a four-game winning streak and is 4-6-0 in its past 10.
"Mad at myself for letting this happen," goalie Marc-Andre Fleury said. "It's tough, as a group, as a team, to cost two coaches their jobs. We have so much respect towards them. They've always been very honest with us, very respectful to the players and you're never going to hear a bad thing about them. They love the game, they love coming to the rink."
McCrimmon said there was no singular reason for the coaching change.
"It's a good wake-up call for all of us," McCrimmon said. "Obviously, it's easier to change two coaches than to change 15 guys. I think we need to hear the message loud and clear and turn things around and be better.
"It wasn't a specific block of games or a specific game. It's hard to put into words, I guess, unless you've done these jobs, but it's more the feeling that you have that a change might be needed. I wish I could be more specific than that, but that's really how we felt. We thought about this a lot; it certainly wasn't something that we did in haste or something that we did based on the recent four games. It was a decision that was arrived at over a period of time.
"It's hard for me to get into specifics. I think that you've seen consistency issues, I think that's part of it. I can't be more clear that that's not necessarily at the feet of the two guys that were relieved today. But I think our team has more to give, and that was what went into the decision."
Gallant was named a coach for the 2020 Honda NHL All-Star Game with Vegas leading the Pacific Division on Jan. 3.
DeBoer, who was fired by the Sharks on Dec. 11, is expected to join the Golden Knights for their game at the Ottawa Senators on Thursday (7:30 p.m. ET; RDS2, TSN5, ATTSN-RM, NHL.TV).
"It's a tough day. We all had a ton of respect for [Gallant] and [Kelly]," forward Mark Stone said. "We take a lot of pride in our dressing room, and the players know that we, as a group, haven't played to expectations, so we're taking quite a bit of that blame. As players, when we don't play up to standards, changes happen. We all loved and respected playing for Gerard, especially the guys that have been here since Day One. I didn't even get a year in with him, but loved playing for him."
DeBoer was 15-16-2 with the Sharks this season and 198-129-34 in five seasons in San Jose, qualifying for the playoffs each season. He is 415-329-111 in 12 NHL seasons with the Sharks, New Jersey Devils and Florida Panthers. He coached New Jersey to the 2012 Stanley Cup Final, when it lost in six games to the Los Angeles Kings, and coached San Jose to the 2016 Stanley Cup Final, when the Sharks lost to the Pittsburgh Penguins in six games. DeBoer also coached the Sharks during their seven-game win against the Golden Knights last season after trailing the best-of-7 Western Conference First Round series 3-1.
"I think Peter's a really good coach," McCrimmon said. "Of course, with him working most recently in the Pacific Division, we've had a chance to watch his teams play, and I think that his teams are always very well-prepared, very well-coached. We expect that he'll do a really good job for us. I think that he's a very respected coach in this industry.
"I think he's a coach that has always had a lot of respect from his players. I think he relates well with players. He's an intelligent guy. [His] teams have always had really strong special teams. He's been to the Stanley Cup Final with two different teams. He's enjoyed a lot of success in the National Hockey League as a coach, and I think with this opportunity he'll continue to do that."
McCrimmon said he believes DeBoer can help Vegas build momentum in the second half of the season and into the playoffs.
"We, like a lot of teams, had real high hopes for the year, still have high hopes for the year," McCrimmon said. "We're in a unique position, we're right in the mix, so this isn't a case where the bottom fell out and we were left with no choice. I guess it was a situation more where, proactively, it was our belief that this is what was the best thing for our organization."
Two of DeBoer's former players with the Sharks said the news caught them off-guard.
"It's kind of shocking," San Jose center Tomas Hertl said Thursday. "Nobody expected the coach from Vegas to get fired, and right away Pete gets there. It's for sure big news and a really big surprise."
Said Sharks forward Timo Meier, "That wasn't the team we expected him to go to, but that's the business. They're a good team, but it's definitely shocking to everybody. It's going to be weird playing against Vegas with him behind the bench."
That reunion will have to wait until next season; Vegas and San Jose have already completed their season series.
This marks the seventh coaching change in the NHL this season. In addition to Gallant and DeBoer, Peter Laviolette was fired by the Nashville Predators on Jan. 7 (John Hynes), Jim Montgomery by the Dallas Stars on Dec. 10 (Rick Bowness), Hynes by the Devils on Dec. 3 (Alain Nasreddine) and Mike Babcock by the Toronto Maple Leafs on Nov. 20 (Sheldon Keefe). Bill Peters resigned as Calgary Flames coach Nov. 29 (Geoff Ward).
NHL.com correspondents Callum Fraser and Rick Sadowski contributed to this report