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.
John Clayton is a senior writer for the NFL for ESPN.com.