With 76 seconds away from defeat and a seventh game, Bryan Bickell tied the game, and 17 seconds later Dave Bolland scored to give Chicago a 3-2 victory.
The back-to-back goals stunned Boston, and with only 59 seconds left the Blackhawks bench began celebrating their championship.
The Blackhawks set an NHL record with a 24-game unbeaten streak to start the shortened season. They won three straight games to come back from 2-1 during the best-of-seven finals.
The sold-out TD Garden arena was nearly empty --except for a few hundred Blackhawks fans -- when NHL commissioner Gary Bettman handed the 35-pound Cup to captain Jonathan Toews, who bounced back from a Game 5 injury and notched a goal and an assist in Game 6.
This is the Blackhawks second NHL championship in four years -- after finishing the lockout-shortened regular season with the league's best record (36-7-5).
Boston, which won the Stanley Cup two years ago, finished the finals with a third straight loss after going up in the series 2-1.