General manager Don Sweeney announced the extension Wednesday, a day before training camp begins. How many seasons the extension covers was not disclosed, though Cassidy reportedly had one-year left on his previous contract.
Cassidy has a 117-74 record with the Bruins, leading them to back-to-back 100-point seasons. The Bruins reached Game 7 of the Stanley Cup Final in June, losing to the St. Louis Blues 4-1.
The 54-year-old former defenseman played 36 games over six seasons for the Chicago Blackhawks. He had a 47-53-9 record in his first head coaching stint, with the Washington Capitals from 2002-04, before he was fired 25 games into his second season.