On Thursday, the Giants traded the outfielder to Toronto in exchange for a player to be named or cash considerations, both clubs announced.
Lewis announced the transaction on his Twitter and Facebook accounts minutes before the club's official announcement.
"Fred is now a Blue Jay!!" he wrote.
Lewis, the Giants' Opening Day left fielder in 2009, batted a career-low .258 (76-for-295) with four home runs and 20 RBIs in 122 games for the Giants last season. The Giants deemed him expendable considering they have six outfielders -- John Bowker, Mark DeRosa, Aaron Rowand, Nate Schierholtz, Andres Torres and Eugenio Velez -- on their roster.
Not helping Lewis' case was injury. He had not appeared in a game this season because of a strained left oblique. He returned from his rehabilitation assignment and was reinstated from the 15-day disabled list on Thursday.
Lewis, who is out of Minor League options, is expected to bring depth to the Blue Jays outfield with his speed and versatility. He has made 225 appearances in left field, 29 appearances in right field and 20 in center field in his career.
"Right now, he's just going to be one of the guys coming off the bench," Blue Jays manager Cito Gaston said. "We'll try to get him in there once in a while, as much as I can."
For his career, the 29-year-old Lewis has a .277 batting average with a .355 on-base percentage along with 16 home runs, 81 RBIs and 34 stolen bases in 326 games over four seasons with the Giants. He was selected by San Francisco in the second round of the 2002 First-Year Player Draft.
"He's a pretty good athlete," Blue Jays general manager Alex Anthopoulos said. "He can run a little bit. He's really just a left fielder in our eyes, but he's someone who we think can be a good left-handed bat off the bench and a guy that, if he's ever thrust into more playing time, at least he's done it before and he's more familiar with that type of role."
To make room for Lewis, the Blue Jays optioned outfielder Jeremy Reed to Triple-A Las Vegas and transferred right-hander Dustin McGowan form the 15-day disabled list to the 60-day disabled list to make room on the 40-man roster.
Jesse Sanchez is a national reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.