Bruce connected for his 25th homer of the season. His sixth home run during his five-game streak broke a 1-all tie in the seventh inning.
The NL West-leading Giants have lost nine of 11 since the All-Star break. The last-place Reds are 8-4 over that stretch.
Dan Straily (6-6) gave up three hits in 7 2/3 innings. Tony Cingrani pitched a scoreless ninth for his 12th save.
Bumgarner (10-6) allowed one earned run and five hits in eight innings, striking out nine.
