The deal prevents Seymour from becoming an unrestricted free agent and makes him the NFL's highest-paid defensive player in terms of annual salary.
"I'm thrilled to be with the Silver and Black, a team that has a bright future," Seymour said via the Raiders' Twitter page. "I see myself retiring a Raider."
Seymour played the 2010 season under a $12.4 million franchise tag and responded with 48 tackles and 5.5 sacks, tied for second-most in his career, in 13 games. He would have been in line for about a $14.9 million salary next season had the Raiders tagged him again, according to The Associated Press.
With Seymour still in the fold, the Raiders can turn their attention to other key players who could become free agents. NFL Network insider Michael Lombardi reported Wednesday on "NFL Total Access" that the team picked up linebacker Kamerion Wimbley's 2011 contract option after he led the team with nine sacks and could use the franchise tag on tight end Zach Miller, its leading receiver the past three seasons, for approximately $7.3 million.
According to Lombardi, the Raiders hope to re-sign All-Pro cornerback Nnamdi Asomugha at a manageable price. The final year of Asomugha's three-year, $45.3 million contract voided earlier this offseason when he didn't reach certain playing incentives, making him one of the Raiders' many potential free agents.
Offensive lineman Robert Gallery, running back Michael Bush, safety Michael Huff, cornerback Stanford Routt and defensive lineman John Henderson also could leave Oakland in free agency.
The Raiders acquired Seymour from the New England Patriots before the 2009 season in exchange for a 2011 first-round draft pick -- a move that some questioned because Oakland's selections recently have been so high. But because the Raiders went 8-8 for their first non-losing season since 2002, the Patriots will pick 17th overall in April's draft.
"You may not think it was a good trade," Raiders owner Al Davis said last month. "I thought it was a great trade. Still do."
Seymour was selected to his sixth Pro Bowl last season, recording 5.5 sacks and anchoring Oakland's defense after moving inside to defensive tackle. Coaches and teammates praised Seymour for his play as well as his leadership on a unit that included two rookies in the front seven.
"It's a great place to play," Seymour said late in the season. "The fans are unlike any others in the league. They're definitely committed, and they want the same type of players to play in this organization. The history of being here, the mystique of putting that silver and black on and representing the Raiders, it's been a lifelong dream for me and, hopefully, it continues."
The Associated Press contributed to this report.