The 31-year-old Ishikawa was hitless in five at-bats for the Giants this season. He began the season on the 15-day disabled list due to a lower back strain.
He hit .266 with nine doubles, four home runs and 20 RBIs in 37 minor league games this season.
This is the second time Ishikawa has been with the Pirates as he made the club out of spring training last season only to be designated for assignment in April. He later signed with the Giants and became a postseason hero for the eventual World Series champions, hitting a game-ending home run against the St. Louis Cardinals in Game 5 of the National League Championship Series.
The Pirates said they would make a corresponding roster move to fit Ishikawa on their 25-man roster once he arrives in Pittsburgh.