The 22-year-old first baseman, who was leading the Pacific Coast League in hitting with a .342 average while racking up 62 RBIs, has six homers with 16 RBI in June.
"Just being around him in spring training, he seemed to be a guy that's very ahead of the game, maturity-wise," Cubs Manager Dale Sveum told MLB.com. "He has a lot of those leadership skills."
Rizzo, who came to Chicago from the San Diego in a January trade, posted a .365 average for Class AAA Tucson last season but didn't hit well during a pair of call-ups to the Padres.
Cubs General Manager Theo Epstein told the Web site Rizzo "was rushed a little bit" by San Diego, accounting for his .141 average and 46 strikeouts in 49 games.
The move came as Chicago, which sports an MLB-worst record of 24-48, was ensnared in a four-game losing streak.