Willis, 33, has not pitched in the major leagues since the 2011 season, when he spent time with the Cincinnati Reds. The deal also includes a potential invite to big league camp this spring.
He has previously signed minor league contracts with the Baltimore Orioles, Chicago Cubs and Los Angeles Angels of Anaheim.
Willis burst onto the scene as a 21-year-old in 2003, when he helped lead the Florida Marlins to an upset win over the New York Yankees in the World Series in six games. That season he finished with a 14-6 record, 3.30 ERA and 142 strikeouts and earned Rookie of the Year honors.
In December 2007, Willis, along with MVP slugger Miguel Cabrera, was traded from Florida to the Detroit Tigers and signed a three-year, $27 million extension. In 2010, he played for the Arizona Diamondbacks, before spending his last year in the majors with the Reds.
In nine seasons, Willis has compiled a record of 72-69 with a 4.17 ERA and 896 strikeouts. He was an All-Star in 2003 and 2005.
- Jack Jorgensen