The team said the 34-year-old Upton will report to extended spring training in Peoria, Arizona.
The Los Angeles Angels released Upton last month. He was in the final season of a five-year, $106 million deal he signed to stay in L.A. a few months after being acquired from the Detroit Tigers via trade during the 2017 season.
A four-time All-Star, Upton was among the game’s promising young stars when he broke into the majors as a 19-year-old with the Arizona Diamondbacks in 2007. He has 324 home runs and 1,000 RBIs over 15 major league seasons.