Matt Grzelcyk scored the Bruins' only goal, and Tuukka Rask stopped 16 shots for Boston.
St. Louis's first title comes in its return to the Stanley Cup Final after 49 seasons.
Ryan O'Reilly isn't just a Stanley Cup champion, he's a Conn Smythe victor, too.
The rookie netminder went legend in Game 7, making 32 saves and improving his record to 9-2 after a loss in the Stanley Cup Playoffs. Three of them have come at St. Louis' expense.
But they knocked it out of the park, landing "fist bump kid" Liam Fitzgerald, along with Olympic gymnast Aly Raisman and New England Patriots wide receiver Julian Edelman as the banner captains ahead of the winner-take-all game.
And, predictably, Bruins fans were pretty excited to see them. Portable party Rob Gronkowski also made an appearance on the video board.
With less than 20 seconds left in the first period, and the Blues charging toward the Bruins' net, Marchand wanted to make a line change and got caught in no man's land.
Bouwmeester played in 1,184 regular-season games - the third most among active players who had not gotten their names etched on the Stanley Cup.