The Packers will receive a 2020 seventh-round draft pick in return, a source told ESPN's Adam Schefter.
The trade comes two days after Montgomery made a special-teams error, running a fourth-quarter kickoff out of the end zone and making a costly fumble in a 29-27 loss to the Los Angeles Rams.
Montgomery, a third-round pick in 2015, switched from receiver to running back in the middle of the 2016 season, but he admitted on Monday that he was unsure of his role with the Packers. Even before he fumbled against the Rams -- a mistake that prevented Aaron Rodgers from attempting a game-winning drive -- Montgomery had played only six snaps from scrimmage. He came off the field after his last play and admitted he expressed some frustration on the sideline.
"I wasn't playing very much, and I wasn't able to make a play and I was frustrated about that," Montgomery said. "I don't fully understand what my role is right now, what I'm supposed to be doing, how I'm supposed to help this team. And it's frustrating when I want to help the team and I get limited opportunities, and it didn't happen on that play. So I came off frustrated about it."
The Ravens' running game has lacked any explosiveness this season with Alex Collins (3.7 yards per carry) and Buck Allen (2.6). Baltimore is the only NFL team that hasn't had a running back break a run over 20 yards.
Montgomery has been one of the more versatile backs in the league as a runner and receiver since being drafted by the Packers. At the very least, Montgomery provides an upgrade over Allen as Baltimore's third-down back.
Montgomery has recorded 1,676 yards from scrimmage (849 rushing on 177 attempts and 827 receiving on 97 catches) during his career, scoring 10 total touchdowns (seven rushing and three receiving).
This is only the third move to add a player at the trade deadline in the Ravens' 22-year history. Baltimore added offensive tackle Eugene Monroe in 2013 and wide receiver Chris Givens in 2015.
The Ravens bring in Montgomery during one of their most critical seasons. Baltimore (4-4) is in third place in the AFC North and is looking to avoid missing the playoffs for a fourth straight season, which could jeopardize the 11-year run of coach John Harbaugh and quarterback Joe Flacco.
Baltimore was in the market for another running back. The Ravens worked out five running backs, including Jamaal Charles, on Friday.
To make room for Montgomery, the Ravens released core special-teams player Albert McClellan.
For the Packers, this cements a two-headed backfield of Aaron Jones and Jamaal Williams. The Packers will have to add another back -- Jones, Williams and Montgomery were the only backs on the 53-man roster at the time of the trade. They had picked up Darius Jackson for the start of the season while Jones was serving a two-game suspension.
ESPN's Rob Demovsky contributed to this report.