He can't play in the Patriots' season-opener because he was not on the roster by 4 p.m. EDT. He is eligible for the Patriots' second game against the Miami Dolphins.
Brown, 31, had an NFL-best 15 receiving touchdowns last season with the Pittsburgh Steelers. The four-time All-Pro and seven-time Pro Bowl selection also had 1,297 yards and 104 catches in 15 starts in 2018.
The Steelers traded Brown to the Raiders in March.
The Patriots host the Steelers at 8:20 p.m. EDT Sunday at Gillette Stadium in Foxborough, Mass.