To make room for Seau on the roster, the team released defensive tackle Terdell Sands, who recorded one tackle in New England's win over Baltimore in Week 4.
Seau had been working for Versus hosting a television show, but the network announced Tuesday that he'd be joining the Patriots.
The 40-year-old will begin his 20th NFL season and his fourth with the Patriots. A 12-time Pro Bowler, Seau has played in 261 games, recorded 1,899 tackles, 56.5 sacks, 18 interceptions, 12 forced fumbles and 17 fumble recoveries.
The signing of Seau and subsequent release of Sands could mean that the Patriots are going to more 3-4 defensive sets in favor of the 4-3 alignment, which they've been somewhat forced into due to injuries. With Jerod Mayo back from a knee injury and the addition of Seau, however, the team has the middle linebackers it needs.