Jackson was informed by head coach Mike Nolan that he was being released. He was scheduled to make $4 million this year. Jackson came to the 49ers in a draft day trade in exchange for a fourth-round draft pick.
"I always had a great deal of respect for Darrell," 49ers coach Mike Nolan said. "I appreciated his dedication and work ethic during his season with the 49ers. He always had a positive attitude every day he came to work. While we are going in a different direction, I know he can still produce and be an asset to an NFL team this year."
He caught 46 passes for 497 yards and three touchdowns in 15 starts last season.
San Francisco signed longtime Rams receiver Isaac Bruce as a free agent last month, adding another veteran who spent his entire career with a division rival. He will play alongside Arnaz Battle and Ashley Lelie in what's been arguably the NFL's least impressive group of pass-catchers for the last few seasons.
Bruce agreed to a two-year, $6 million deal two weeks ago.
The 49ers have fruitlessly searched for good veteran receivers during Nolan's three seasons with the club, with a spate of dismal signings that include aging standouts Johnnie Morton and Curtis Conway.
San Francisco also got one tumultuous season from Antonio Bryant, who performed fairly well but earned a four-game suspension from the NFL late in the 2006 season, prompting the 49ers to dump him.
Last spring, they signed Lelie and took a chance on Jackson in a trade with Seattle for a fourth-round pick. Neither deal has worked out, with Lelie struggling to get playing time behind Battle and Jackson.
Senior writer John Clayton covers the NFL for ESPN.com. Information from The Associated Press was used in this report.