Zambrano will report this week to Clearwater, Fla., for extended spring training. The three-time All-Star hasn't pitched in the majors since last September with Miami.
Zambrano, 31, is 132-91 with a 3.66 ERA in 12 seasons, including 11 with the Chicago Cubs, averaging 14 wins with a 3.43 ERA from 2003-10. He was 16-17 with a 4.66 ERA in the past two seasons.
The Phillies were looking to add to their pitching stockpile after losing veteran Roy Halladay to a shoulder injury. John Lannan, another member of the team's Opening Day rotation, is expected to be out until June with a knee injury.
"We're just looking to add some starting pitching depth and some experience," said Phillies assistant general manager Scott Proefrock. "No promises were made. It's low-risk and hopefully high-reward."
The Phillies ramped up their pursuit of Zambrano after watching him throw Monday in Miami. Jorge Velandia, a minor league instructor with the Phillies, watched Zambrano in his role as an official with Team Venezuela in the World Baseball Classic, and passed along a positive report.
The Phillies signed Zambrano with ample knowledge of his history of combustibility and controversial incidents with the Cubs. The deal includes an out clause for Zambrano in early July if the Phillies don't add him to the big-league roster by then.
"We got positive feedback from everybody who has interacted with him recently,'' Proefrock said. "We did our due diligence. It's just good business.
"We got a feel for his desire to come back and his sincerity about that, and what kind of condition he's put himself in. It's a minor league deal. We're not signing him to a $90 million deal like the Cubs did, but you still want to get a guy who's committed and is going to conduct himself appropriately. As far as we're concerned, that was satisfied."
Information from ESPN.com senior writer Jerry Crasnick and The Associated Press was used in this report.