Scott Rolen tripled and scored the go-ahead run on an eighth inning Todd Frazier groundout as the Reds won their fifth straight game and extended their NL central lead.
The Reds have won 15 of 16 and now lead Pittsburgh by 5 1/2 games in the division.
Jonathan Broxton pitched the eighth inning for his first win with the Reds. Aroldis Chapman pitched out of a 9th inning jam for his 25th save.
The Reds can sweep the Pirates with a win on Sunday.