Cassidy's current deal expires after the 2019-20 season. He took over behind Boston's bench in February 2017, replacing Claude Julien. Cassidy joined the NHL club as an assistant coach prior to the 2016-17 campaign after working with the AHL's Providence Bruins from 2008-16.
The Bruins own the NHL's second-best record (117-52-22) during the 54-year-old's tenure. Only the Tampa Bay Lightning have been more successful over that span.
Boston has qualified for the playoffs in all three seasons under Cassidy and came within one victory of winning the Stanley Cup this past spring.