Quiroz, batting for pitcher Santiago Casilla (3-2) with one out in the bottom of the 10th, lined a pitch from Brandon League (0-1) into the left field stands, giving the Giants their second walk-off homer in two games.
Buster Posey homered to end Friday's 2-1 victory over the Dodgers in the first of the three-game series.
Matt Kemp and Dee Gordon each drove in a pair of runs and A.J. Ellis added a bases-empty homer for the Dodgers, who dropped a third consecutive contest.
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