It was the fifth win in six games for the Bruins, who were blanked over the first two periods before scoring twice in a span of less than 2 minutes.
Seguin scored on a backhander just 14 seconds into the final period and Krejci tipped in a shot by Milan Lucic to put the Bruins in front.
Those two quick goals made a winner out of Tuukka Rask, who stopped 20 shots. Boston has suffered only two losses this season and one of those came in overtime.
The lone Montreal goal came from P.K. Subban midway through the second period. The power-play tally was his first of the year.
Carey Price made 23 saves for the Canadiens, who had won six of their previous seven.