 On May 24, 2016, Cassidy joined the Boston Bruins as assistant coach for the 2016–17 season.On February 7, 2017, he was named interim head coach after head coach Claude Julien was fired, and on April 26, the Bruins named him the new head coach.In his first full year as Bruins head coach, he led the team to a 50–20–12 regular season record, collecting 112 points and securing the second seed in the Atlantic Division.