The move means all of the Raiders' veterans are under contract for next season. The team announced the signing on its website, adding that the process of signing rookies would begin soon.
By placing the tag on Seymour in February, the Raiders guaranteed Seymour a salary for next season of at least $12.398 million, which was the average salary earned last year by the five highest-paid defensive ends in the league.
Seymour, 31, was not allowed to negotiate with other teams so it was inevitable that he would return to the Raiders. Coach Tom Cable said the sides had been working on a long-term deal during the offseason but Seymour signed the one-year deal instead.
Because Seymour was not under contract, he missed Oakland's entire offseason program. He will rejoin the team when training camp starts in Napa on July 28.
The Raiders acquired Seymour just before the start of last season by sending a 2011 first-round pick to New England. He was paid about $3.7 million last season.
Seymour was a stalwart in New England after being drafted sixth overall in 2001. He played on three Super Bowl winners with the Patriots, recording 39 career sacks and being selected to the Pro Bowl for five straight seasons beginning in 2002.
Seymour made an impressive debut just days after arriving in Oakland, recording two sacks in a season-opening loss to San Diego. But Seymour had only two more sacks in the final 15 games and was unable to solve the Raiders' run defense woes.
The Raiders finished the season 5-11, their NFL-worst seventh straight season with at least 11 losses. Oakland finished 29th in run defense at 155.5 yards per game.
The Raiders have made some key moves to upgrade the run defense this offseason. They used their first two draft picks on middle linebacker Rolando McClain and defensive end Lamarr Houston. They also traded for linebacker Kamerion Wimbley and signed two-time Pro Bowl defensive tackle John Henderson earlier this month.