The Braves said Howard will report to extended spring training next week, then join Triple-A Gwinnett.
Howard, 37, spent 13 seasons with the Phillies before an emotional farewell at the end of last season. He was the last remaining member of Philadelphia's 2008 World Series-winning team, and the Phillies used a $10 million buyout on his contract this offseason.
The 2006 National League MVP indicated he wasn't planning to retire, saying during his farewell speech that "I know there's more in the tank."
I'm excited and energized to get going and start this new chapter!!!!#atlantabraves— Ryan Howard (@ryanhoward) April 6, 2017
With the Braves, Howard could offer insurance for first baseman Freddie Freeman or serve as a designated hitter call-up for interleague games.
If added to the 40-man roster, Howard would get a contract paying a $750,000 salary in the major leagues this year and $120,000 in the minors. He would have the chance to earn $750,000 in performance bonuses based on plate appearances: $150,000 each for 100, 175, 250, 325 and 400.
"You can't ever have enough players, because it takes a lot of guys to get through this long season," Braves manager Brian Snitker said. "Just from being across the diamond and seeing him at some events in the offseason, [he] seems like a great guy and a good teammate."
Freeman, who started 158 games at first base last season, applauded the acquisition of Howard as a potential impact bat off the bench for the Braves.
"Obviously he has something left in the tank,'' Freeman told reporters at Citi Field before Thursday's game between the Braves and New York Mets. "We all saw it in the second half last year. If he can get some at-bats and prove that he can still do it, why not?''
After hitting .154 in the first half with the Phillies in 2016, Howard picked up his production after the All-Star break -- hitting .262 with a .932 OPS.
Freeman said he discussed the possibility of signing Howard with general manager John Coppolella during spring training and gave the move his full endorsement.
"If he has something left, we'll gladly take him, because we need him,'' Freeman said. "We have a lot of versatile guys on the bench. But he's a guy who can still change the game with one swing of the bat.''
Howard is a career .258 hitter with 382 home runs and 1,194 RBIs. He tied for the Phillies lead in 2016 with 25 home runs but also hit a career-worst .196.
"There was no real risk on our end," Coppolella said. "He's a great makeup guy. He's good with young kids. If it works, it's great. If it doesn't, it's not a big league contract."
ESPN's Jerry Crasnick and Buster Olney and The Associated Press contributed to this report.