The appointment is expected to be made official Friday, the same day Montgomery's contract as an assistant coach with the St. Louis Blues expires.
The 53-year-old Montgomery replaces Bruce Cassidy, who was fired by the Bruins on June 7 after six seasons behind the bench. Cassidy has since been named head coach of the Vegas Golden Knights.
The Boston gig will be Montgomery's second NHL head-coaching job. He was hired by Dallas in May 2018 and immediately took the Stars back to the playoffs for the first time in three years. But Montgomery's tenure in Dallas didn't last long.
On Dec. 10, 2019, he was fired by the Stars for "unprofessional conduct inconsistent with the core values and beliefs of the Dallas Stars and the National Hockey League." General manager Jim Nill said there was a "material act of unprofessionalism" that led to Montgomery's firing, without elaborating further.
On Jan. 3, 2020, Montgomery announced that he was checking into rehab for alcohol abuse.
In September 2020, the Blues hired Montgomery to serve as an assistant under head coach Craig Berube. Montgomery was primarily involved in special teams, helping St. Louis to rank second overall (25.5%) on the power play and 10th (81.3%) on the penalty kill the past two seasons.
Montgomery was a player himself, appearing in 122 NHL games from 1993 to 2003 between the Blues, Montreal Canadiens, Philadelphia Flyers, San Jose Sharks and Stars. He recorded nine goals and 34 points.
After hanging up his skates, Montgomery made several successful coaching stops prior to rejoining the NHL ranks.
He was head coach of the United States Hockey League's Dubuque Fighting Saints from 2010 to '13, guiding the club to league championships in 2010-11 and 2012-13.
In 2013, Montgomery was named head coach at the University of Denver and led the Pioneers to a national championship in 2017. Montgomery was also voted the national coach of the year for the 2016-17 season.
Boston is coming off its sixth consecutive playoff appearance, which ended in a first-round loss to the Carolina Hurricanes. Montgomery will be the 29th coach in the Original Six's franchise history.