Ishikawa, 31, was 0 for 5 in six games for the Giants this season. He also hit a combined .266 with four home runs and 20 RBIs in 37 games with Triple-A Sacramento and Class A San Jose.
Ishikawa was the Pirates' opening day first baseman last season, but was designated for assignment April 19.
He signed with the Giants six days later and hit a game-ending home run against the St. Louis Cardinals in the decisive Game 5 of the NLCS to give San Francisco its third pennant in five seasons.
The Pirates will make another roster move once Ishikawa reports.