Victorino, obtained from Philadelphia at the July 31 non-waiver trade deadline, smacked an RBI double in the first inning to open the scoring and came home on Matt Kemp's single.
Then, with the score tied 2-2 in the fifth, pitcher Aaron Harang collected a one-out single and Victorino followed by lining a 3-2 pitch into the right field bleachers for a 4-2 lead.
Juan Rivera chased home Kemp with an eighth-inning single in what turned out to be the decisive run.
Harang (8-7) earned the victory, giving up a pair of runs on five hits over six innings. He walked three and struck out four.
Ronny Belisario surrendered two runs on three hits in the eighth to make it close, but Kenley Jansen turned in a 1-2-3 ninth for his 24th save.
Pittsburgh's Jeff Karstens (4-3) was touched for four runs on seven hits over seven full frames.
Garrett Jones went 4-for-4 with three RBI for the Pirates, whose lead over the Dodgers for a National League wild card spot shrank to 1 1/2 games.