Jonathan Quick made 29 saves for the Kings, who are off to their best start in franchise history at 9-1-1.
Brad Marchand scored for the Bruins and Tuukka Rask, playing his first game after missing the previous three with a concussion sustained in practice, stopped 28 shots.
With 0.9 seconds left and the puck dropped for a faceoff, Anze Kopitar won the draw back to Toffoli, who unloaded a shot that beat Rask to the far post from the top of the right circle, stunning the Bruins and the TD Garden Crowd that seemed ready for a shootout.
The clock had been reset by the referees from 0.4 to 0.9 before the faceoff.
The Kings’ celebration was delayed by a replay review, but they mobbed Toffoli after it counted.
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