Tucker Barnhart added a run-scoring triple in the sixth as the Reds sent the Pirates to their fifth straight loss and 10th in 11 games. The loss guarantees Pittsburgh's second straight losing season after three consecutive years of at least 88 wins.
The win was Cincinnati's 66th of the season, two short of matching last season's total.
The Pirates, who've scored a total of nine runs in their last five games, got just one hit against Stephenson (5-5) — pitcher Gerrit Cole's double in the third.
After Deck McGuire pitched a one-hit seventh for Cincinnati, John Jaso hit a two-run homer off of Luke Farrell in the eighth to avoid a shutout. Michael Lorenzen pitched a perfect ninth for his second save of the season.
Cole (11-11) took the loss.
