Jennings, an eight-year NFL veteran, was the first major free-agent signing of former coach Mike Nolan's tenure in 2005, but the lineman played in just 23 of the 49ers' 64 games, finishing three of his four seasons in San Francisco on injured reserve.
Although signed to a seven-year, $36 million contract to be the 49ers' left tackle, Jennings missed all but three games of his first season in San Francisco with shoulder injuries. He played in just five games in 2007 while dealing with an ankle injury and undisclosed personal issues.
Jennings would have made $4.2 million in base salary during the 2009 season, but the 49ers told him to stay away from their minicamp earlier this month while they worked out the details of releasing him.
Jennings moved to right tackle last season, but he played in just two games before dislocating his right shoulder, which required three major surgeries during his years with the 49ers.
Jennings was mostly healthy in his four seasons with the Buffalo Bills, playing in 11 or more games in all four seasons (2001-04) with the team.
Jennings will be replaced by Smith, whom the The Sacramento Bee reported Friday agreed to a two-year contract with the 49ers. Smith, a 2005 Pro Bowler who has dealt with his own injury problems, still started 12 games or more in seven of his nine seasons with the Pittsburgh Steelers.
The Associated Press contributed to this report