This will be Ishikawa's second stint with the Pirates, as he made the team out of Spring Training last year as a non-roster invitee.
Pittsburgh designated him for assignment on April 19, 2014, after he hit just .206 in 15 games. He signed a Minor League deal with San Francisco less than a week later.
"We've liked the player. It wasn't an easy move when we moved him out before," Pirates manager Clint Hurdle said. "He's done some nice things since then, got some experience. He's got a left-handed bat, can play some first base, can play some outfield. I do think it's a guy that can help our club."
Ishikawa spent the rest of the season with the Giants, hitting the walk-off home run that sent them to the World Series in Game 5 of the National League Championship Series.
Ishikawa, 31, went 0-for-5 in six games with the Giants this year after opening the season on the disabled list. He hit a combined .266/.335/.413 with four home runs and 20 RBIs for Class A Advanced San Jose and Triple-A Sacramento this year.
Ishikawa is a career .257/.321/.395 hitter over 450 games in the Majors.