NBC Bay Area Sports broke the story during the weekend and now it is confirmed by the team.
Raiders senior executive John Herrera told The Associated Press that the four-time Pro Bowler will join the club's off season workout program immediately alongside starter JaMarcus Russell.
Garcia started his NFL career with the 49ers after playing for the Canadian Football League's Calgary Stampede. He was a standout at San Jose State.
The Gilroy-native spent the past two seasons with the Tampa Bay Buccaneers, starting 24 games and leading them to the playoffs, but wasn't re-signed by the club.
The 39-year-old quarterback enjoyed the best seasons of his NFL career with the San Francisco 49ers, passing for more than 16,000 yards from 1999-2003.