The Marlins' rotation has struggled of late. Brad Hand and Jacob Turner are currently part of the five-man staff, but both have yet to solidify their spots.
Penny, 36, last pitched in the big leagues in 2012, appearing in 22 games with the Giants. The right-hander was used in relief in each of his outings with San Francisco, but he's been primarily a starter in his 13-year career.
Penny broke in with the Marlins in 2000, and he was part of the 2003 World Series championship team. He last pitched for the Marlins in 2004, before being traded to the Dodgers midseason as part of the Paul Lo Duca deal.
"He's a veteran with experience," Marlins president of baseball operations Michael Hill said. "If we think he can help us, we signed him for a reason. We'll continue to evaluate him, and see where he falls in line."
Penny has a career mark of 119-100 with a 4.26 ERA. He's a two-time All-Star, as well as a former teammate of Miami manager Mike Redmond.
"Brad knows what's going on," Redmond said. "If that day comes, we'll cross that bridge when we get there."
Redmond also made it clear that performance is demanded from the starters.
"I think at this point, we haven't ruled anybody out," Redmond said. "We need quality starts. We need guys to give us a chance. But right now, we're going to run with the guys that we have."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter