With rosters expanding, Chicago recalled pitcher Tsuyoshi Wada and selected the contracts of outfielder Quintin Berry and reliever Trevor Cahill from Iowa. To make room on the 40-man roster, the Cubs designated relievers James Russell and Rafael Soriano for assignment.
Catcher David Ross was activated off the family medical emergency list.
Outfielder Austin Jackson, acquired Monday from Seattle, was added to the active roster.
"You've got a good mix of veterans and young guys," Jackson said. "That's a good mix that you need when you're approaching the postseason."
Baez, 22, is one of Chicago's top prospects but struggled during a brief stint in the major leagues last year. In 52 games, Baez hit nine home runs but struck out 95 times and batted.169.
Since then, Baez has changed his approach and worked to eliminate a violent leg kick from his swing in hopes to make more contact.
"That's been a process for him," Cubs president Theo Epstein said. "It's something that was gradual."
This season at Iowa before being recalled, Baez hit .324 with 13 home runs in 70 games. Baez, who was batting seventh and playing second base, missed time this season after a wrist injury and the death of his sister.
"It's been a really tough year for me and my family and plus my injury I was out for almost two months," Baez said. "That's what makes me really patient."
Though he started at second base, Baez could see time all over the field.
"His time's finally arrived to be up here," Cubs manager Joe Maddon said. "I'm sure he's going to handle it properly."
Wada, 34, has spent parts of the last two seasons with the Cubs, going 5-5 with a 3.40 ERA.
Cahill, 27, has also pitched for Oakland, Arizona and Atlanta. Signed to a minor-league deal by Chicago on Aug. 18, Cahill appeared in five games without allowing a run at Iowa.
Berry, 30, has played for Detroit, Boston and Baltimore before being signed by the Cubs on Aug. 24. He appeared in five games at Iowa, going 2 for 14.
Russell was 0-2 with a 5.29 ERA in 49 appearances, while Soriano went 2-0 with a 6.35 ERA in six games.