Braves manager Fredi Gonzalez said Jackson will work as a reliever. Gonzalez told Jackson he has the next seven weeks to use as an audition for next year. Jackson turns 32 next month.
Atlanta's bullpen ranked third-worst in the majors with a 4.24 ERA going into Friday night's game against Arizona.
Jackson signed a four-year, $52 million contract with the Chicago Cubs in January 2013. He went 14-33 with a 5.58 ERA in his first two years with the Cubs and was 2-1 with a 3.19 ERA in 23 relief appearances before they released him last month.
The Braves will pay Jackson the prorated amount of the major league minimum this year. The Cubs are responsible for Jackson's $13 million salary in 2016, minus the major league minimum if he pitches for another team.