He was selected by the Boston Bruins with the 2nd overall pick in the 2010 NHL Entry Draft. He played 2 seasons in the OHL with the Plymouth Whalers. Seguin has also played in the NHL for the Boston Bruins for 3 seasons. During his time with the Bruins, he won the Stanley Cup on June 15, 2011 when they defeated the Vancouver Canucks in game 7 by a score of 4-0. On July 4, 2013, he was traded by the Bruins along with Rich Peverley, and Ryan Button to the Dallas Stars for Loui Eriksson, along with Reilly Smith, Matt Fraser, and Joe Morrow.