In another move Sunday, the Patriots signed veteran free-agent guard Kendall Simmons to a three-year deal, reports NFL Network's Jason La Canfora.
Seymour is in the final year of a contract paying him $3.7 million this season. In return, the Patriots received a first-round draft choice in 2011.
"From nearly the day he arrived in 2001, Richard Seymour established himself as one of our premier players for nearly a decade," Patriots coach Bill Belichick said Sunday. "His presence has been felt as a force on the field, a respected man off it and a multiyear champion."
The move is only the latest as the Patriots continue to get younger on defense. Linebacker Mike Vrabel was traded to Kansas City in the offseason, safety Rodney Harrison announced his retirement, and more recently Tedy Bruschi officially called it a career after 13 seasons.
"Any transaction we make is with the goal of what is best for our team and, as difficult as it is to part ways with a player of Richard's stature, many factors were taken into account when we considered this trade," Belichick said.
"As an organization, we feel the trade with Oakland brings sufficient value and is in the long-term interest of the club," he added. "We are extremely grateful for the huge impact Richard's elite level of performance had on our success and we wish him the very best during the rest of his career."
The addition of rookies Myron Pryor and Ron Brace to a defensive line that also includes veterans Vince Wilfork, Ty Warren, Jarvis Green and Mike Wright made Seymour expendable.
"It's the nature of the business," said veteran running back Kevin Faulk, the longest-serving player on the team after Bruschi's departure. "They have an agenda upstairs, and the coaches decide who comes and goes. You can't do nothing about it."
Faulk said the team will miss the way Seymour carried himself as a person and a player.
"He was a quiet leader," second-year linebacker Jerod Mayo said. "He led by his actions. He'll be dearly missed as a teammate and as a player on the field. But we have players that are ready to step up."
When asked if the Pats might be better without Seymour, Mayo said: "Ask me after Game 1."
Simmons, 30, was a first-round pick by Pittsburgh in 2002 and he won two rings with the Steelers. He has been a starting guard his entire career when healthy (he missed all of 2004 due to injury) and is coming off Achilles surgery that ended his 2008 season just four games in (he was released by the Steelers in February). He's played 80 regular-season games, all starts.
The Associated Press contributed to this report