Sources told SiriusXM NFL Radio that it's a one-year deal for Willson. The team didn't release financial details of the contract.
The signing of Willson comes a few days after former Oakland tight end Jared Cook left the franchise and signed with the New Orleans Saints. Cook was one of Raiders quarterback Derek Carr's favorite targets, bringing down 68 catches for 896 yards and six touchdowns in 16 games (14 starts).
Willson joins the Raiders after spending last season with the Detroit Lions. He had 13 receptions for 87 yards in 14 games with the team.
The Seahawks originally drafted Willson in the fifth round of the 2013 NFL Draft. He played in five seasons with Seattle and tallied 89 catches for 1,129 yards and 11 scores.
Willson has played in 86 career games (45 starts) and has recorded 102 catches for 1,216 yards and 11 touchdowns over his six-year NFL career. In nine career playoff contests, he has 13 receptions, 154 yards and one touchdown.