Douglas, 30, becomes the ninth new addition to the Bucs, who continue to work on other deals. Douglas, a 16-game starter for the 49ers, worked in a 4-3 defense in Baltimore and a 3-4 defense in San Francisco.
Under defensive coordinator Monte Kiffin, the Bucs use a 4-3 defense.
The 6-2, 292-pound Douglas has started 78 out of a possible 80 games over the past five years for both the Ravens and the 49ers. He left the Ravens in 2005 and signed a three-year deal with the 49ers as one of Mike Nolan's first key acquisitions in trying to turn around that team.
Douglas joins defensive end Jimmy Wilkerson and linebackers Teddy Lehman and Leon Joe as additions to the Bucs' defense. Tampa Bay's other five moves were on offense.
John Clayton is a senior writer for the NFL for ESPN.com.