The Senators (22-37-5) have lost six straight games and are last in the NHL standings.
"I want to thank Guy for his three years of service," Ottawa general manager Pierre Dorion said. "He is a good person and has been an excellent representative of the Senators. At this point, however, we need a different set of coaching and leadership skills to guide our team through this rebuild. In the interim, Marc will bring a different perspective along with a wealth of head coaching experience."
Video: The crew discusses the Senators firing Guy Boucher
Dorion said he made the decision after a 4-2 loss to the Edmonton Oilers at Canadian Tire Centre on Thursday and informed Boucher after discussing the situation with owner Eugene Melnyk. Dorion met with the players before practice Friday.
"I told them that recently our play wasn't acceptable, and that moving forward I didn't want any excuses," Dorion said. "I think we're fielding a team from now to the end of the year that can win games. And we want to see this team win games, develop, grow, have a great culture, play for each other, play for the coaching staff, and more importantly play for our fans. And we saw that against Calgary a few days ago (a 2-1 loss Sunday). I don't think we saw that last night.
"And at a certain point in time, we felt that we had to make this decision today. And it's a decision that tries to set a foundation for this organization and this rebuild to thrive as earliest as possible."
Dorion said Crawford will remain coach for the remainder of the season and will be a candidate for the full-time job. The 58-year-old's first game as coach will be against the Tampa Bay Lightning at Amalie Arena on Saturday (7 p.m. ET; CITY, TVAS2, SUN, NHL.TV).
"I joked with him that if he goes 18-0, he's got the job," Dorion said. "But at the same time, we're going to do a very extensive search. We did it last time. We have an idea of the direction we want to go in. … I think a communicator is very important, someone who listens. A teacher, someone who has structure, and someone who can really expedite this rebuild as quickly as possible."
Crawford was in his third season as associate coach of the Senators. He previously has coached the Quebec Nordiques, Colorado Avalanche, Vancouver Canucks, Los Angeles Kings and Dallas Stars. In 1,151 games, he has a record of 549-421-78 with 103 ties. He coached the Avalanche to the Stanley Cup in 1996 in their first season in Colorado and won the Jack Adams Award in 1995 with Quebec in his first season as an NHL coach.
Crawford said player development coach Chris Kelly has been added to the coaching staff, joining assistants Rob Cookson and Martin Raymond, goaltending coach Pierre Groulx and video coach Kristopher Young.
"We really do have a great group of players here," Crawford said. "Yeah, we're not showing it in the standings, but they're a group that care for one another, they're a group that, emotionally, is as good as any Stanley Cup team that I've been around, and the measure of the personalities in that room is extraordinary. These are young guys, really young. And they are in a situation that is very, very difficult."
The Senators have traded their top five scorers from last season, including centers Matt Duchene (on Feb. 22) and Ryan Dzingel (on Feb. 23) to the Columbus Blue Jackets in separate trades, and forward Mark Stone to the Vegas Golden Knights prior to the NHL Trade Deadline on Monday. Forward Mike Hoffman was traded June 19 to the San Jose Sharks, who then traded him to the Florida Panthers the same day, and defenseman Erik Karlsson was traded to the Sharks on Sept. 13.
The returns on the trades mostly have been prospects and draft picks, and Dorion told NHL.com the future of the Senators will be built around a group of young players already on the roster, among them 22-year-old defenseman Thomas Chabot and 19-year-old rookie forward Brady Tkachuk.
"We now have 17 picks in the first three rounds over the next three NHL Drafts (2019-21)," Dorion told NHL.com on Tuesday. "Our fans have seen some roster players we acquired in those deals -- specifically the Erik Karlsson one -- in, for example, forward Chris Tierney and defenseman Dylan DeMelo. They had seen some prospects coming in in (forwards) Rudolfs Balcers and Josh Norris. There are up-and-comers on our Belleville (American Hockey League) farm team in (forwards) Drake Batherson and Logan Brown. And there are the young guys we have up here in (forward) Colin White and, of course, what can you say about Brady Tkachuk and Thomas Chabot? We feel they are going to be stars."
Boucher was 94-108-26 in 228 games with the Senators. Ottawa reached the Eastern Conference Final in 2016-17, his first season, and lost Game 7 to the Pittsburgh Penguins in the second overtime. Ottawa missed the playoffs last season, finishing 30 points behind the New Jersey Devils for the second wild card from the Eastern Conference, and is all but certain to miss them again this season.
In six seasons with the Senators and Lightning, the 47-year-old is 191-187-46 in 424 games.
Boucher is the seventh coach to be fired this season, following Randy Carlyle (Anaheim Ducks), Dave Hakstol (Philadelphia Flyers), Todd McLellan (Edmonton Oilers), Mike Yeo (St. Louis Blues), Joel Quenneville (Chicago Blackhawks) and John Stevens (Los Angeles Kings).
Dorion had told the Ottawa Sun on Thursday that a decision on Boucher, who was in the final season of his contract, would be made after the season.
"Guy's job hasn't been easy in the last few weeks with everything going on and I think he's handled it well," he said then. "We're going to evaluate them at the end of the year."
NHL.com correspondent Callum Fraser contributed to this report