The signing is expected to become official Wednesday, the start of the 2019 NFL league year. The league's free agency legal tampering period began at noon Monday.
Brown, 25, started 16 games in his first season with the Patriots in 2018. The seventh round pick in the 2015 NFL Draft played his first three seasons with the San Francisco 49ers before being traded to the Patriots for a third round pick in April.
The move continues a spending spree for the Raiders, who agreed to a trade for Antonio Brown on Saturday. The deal for the former Pittsburgh Steelers pass-catcher also doesn't become official until Wednesday. The Raiders also reworked the All-Pro's contract, giving him a three0year pact with a max value of $54.125 million.
Brown rated No. 32 among tackles in 2018, according to Pro Football Focus grading.