Jackson, who turns 32 next month, will pitch out of the bullpen, manager Fredi Gonzalez told reporters. Jackson was signed for the remainder of this season.
The Chicago Cubs released Jackson in July. He signed a four-year, $52 million deal with Chicago in January 2013.
Jackson was 14-33 with a 5.58 ERA in his first two years with the Cubs, working almost exclusively as a starter. He found new life as a reliever this season, appearing in 23 games and posting a 3.19 ERA before being released.
The Braves could use Jackson's help in the bullpen. Atlanta's relievers have posted a 4.24 ERA, third-worst in the majors entering Friday.