The signing gives the 48-year-old Franco a chance to continue pursuing his dream of playing until he is 50, which he expressed when designated for assignment July 12 by the New York Mets. He turns 49 next week.
"He's playing (Thursday)," said Braves Manager Bobby Cox on the team's Web site after Atlanta lost its third game in a row to Cincinnati, 5-4 in 15 innings Wednesday.
Atlanta begins a four-game series against St. Louis Thursday.
Franco, the oldest player in the majors this season, was batting .200 with one home run and RBI in 50 at-bats when he was released.
He has a .298 career batting average with 173 homers and 1,186 RBI.