On May 25, 2015, Peralta hit a walk-off home run in the tenth inning against the Diamondbacks, his seventh home run of the season. In his time with the Cardinals through June 1, he ranked first in extra-base hits (80) and home runs (29), third in RBI (101), second in slugging percentage (.461) and OPS (.804) and fourth in on-base percentage (.361) among MLB shortstops. On defense, he did not rely heavily on range, but on smart positioning and making the routine plays, qualifying him for +1 defensive runs saved after saving 17 the year before.
He was selected as the National League's starting shortstop in the All-Star Game at Great American Ball Park in Cincinnati after batting .298 with 13 home runs and 46 RBI over the first 87 games in 2015. After the All-Star break, he batted .243, and produced three doubles and two home runs over the final two months, or 54 games. Peralta finished the season with a .275 batting average, 17 home runs, 71 RBI, 26 doubles, and 159 hits in 155 games played.