Smith, 29, was drafted in the fifth round by the Ravens in 2007 and played for the team for three seasons.
In four NFL seasons, including one with the San Francisco 49ers where he won three of five starts, Smith has passed for 1,734 yards, eight touchdowns and five interceptions for a 78.5 passer rating. He passed for 558 yards in 14 games with the Ravens.
Smith won a Heisman Trophy at Ohio State, where he passed for 5,720 yards and 54 touchdowns.
"We welcome Troy to the Alouettes," Alouettes general manager Jim Popp said in a statement. "We've been in discussions together for well over three months and have been patiently waiting for Troy to finish his commitments at Ohio State.
"We'll have to fast track Troy in order to get him over the learning curve of the CFL and caught up to speed over the next several weeks. He's here to learn and grasp the game for future competition."
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