The Padres were in last place in the National League West with a 42–76 record, 26 games out of first place, and traded Miller to the National League East division-leading Pittsburgh Pirates on August 10, 1971 for Johnny Jeter and Ed Acosta. Miller continued his success in the bullpen, ending the season with the Pirates with a 1–2 record and three saves in 16 appearances, all in relief, finishing with a 1.29 ERA. Miller pitched the second game of the 1971 National League Championship Series, entering in the sixth inning and giving up three hits, three walks and two runs in three innings of work in a game in which the Pirates held on to defeat the San Francisco Giants by a final score of 9–4. He pitched in relief in three games in the 1971 World Series against the Baltimore Orioles that the Pirates won in seven games, appearing in relief in all three Pirate losses (in the first, second and sixth games of the series), taking the loss in Game 6 when he gave up a sacrifice fly to Brooks Robinson that brought in the winning run.