The 27-year-old right-hander signed with the Giants as a Minor League free agent before this season and had spent the entire 2012 season in Fresno's starting rotation, going 7-7 with a 3.46 ERA. Petit last appeared in the Majors with the D-backs in 2009 and will provide another option in a deep but recently worn out relief corps.
"He can help give us innings," manager Bruce Bochy said. "He's more of a long guy because he's been starting, and he gives us coverage there if something happens. He can start or eat some innings there. He's got experience there, and he was throwing the ball well down in Fresno. We just wanted to give ourselves more coverage."