Williams will receive a four-year deal worth $44 million, with a max value of $47 million and $22 million in guarantees, added Schefter, citing a source.
The 27-year-old adds size and athleticism to Oakland's improved receiving corps. The team recently completed a blockbuster trade for former Pittsburgh Steelers star Antonio Brown.
Williams caught 41 passes for 653 yards and five touchdowns in 2018. He thrived in a larger role during the 2016 season, accumulating 1,059 receiving yards.