On March 2, 2015, during the 2014–15 season, Petry was traded to the Montreal Canadiens in exchange for a 2015 second-round draft pick and conditional fifth-round pick. After the Canadiens post-season run, on June 2, 2015, Petry opted to surrender his impending free agent status in signing a six-year contract extension with the Montreal Canadiens until the 2020–21 season.