Bumgarner (7-11) scattered four hits over seven scoreless innings, issuing only one walk and fanning 10 batters for the Giants.
Sergio Romo and Santiago Casilla finished up the whitewash with one inning apiece in relief and San Francisco remained one-half game ahead of second-place Arizona in the National League West with its second victory in six games.
Aubrey Huff went 3-for-4 with a solo homer and RBI double for San Francisco.
James McDonald (7-6) was touched for three runs on four hits over six frames for Pittsburgh, which lost for the 11th time in 12 contests.
Notable deaths of 2014 [PHOTOS]