To acquire Lynch, Oakland sent Seattle its fifth-round draft pick in the 2018 draft while also getting the Seahawks' 2018 sixth-round pick in return.
Lynch and the Raiders have an agreement for a two-year contract in place, league sources told ESPN's Adam Schefter.
Lynch, who retired after the 2015 season, was born and raised in Oakland. The Raiders will play there for at least the next two seasons before moving to Las Vegas.
Raiders quarterback Derek Carr said he was looking forward to Lynch joining the Raiders.
"He's one heck of a football player, and I hope to play with him. ... Of course, anyone would want Marshawn Lynch, any quarterback in the NFL," Carr said last week. "As we add pieces, we'll hug them up and bring them in just as a family and treat them the same way that we do with all of our other teammates."
Defensive end Khalil Mack had high praise for his future teammate, calling him the most physical player he has faced.
"Marshawn is one of the best running backs I've ever played against in my short career so far," Mack said. "You talk about a player, a hell of a player. It would be a great, great, great treat to have him on this side, on the dark side."