Seguin's overtime winner pulled the defending Stanley Cup champions even with the Capitals at three wins apiece as the Eastern Conference quarterfinal series now shifts back to Boston for a deciding Game 7 Wednesday.
Seguin also logged an assist as the Bruins shook off a 4-3 loss in Saturday's sixth game.
Tim Thomas turned aside 36-of-39 shots to earn the victory.
Caps goaltender Braden Holtby yielded four goals on 31 shots.
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