The deal is worth $17 million, with $9.5 million guaranteed, sources told ESPN's Todd Archer. A source told ESPN's Adam Caplan that the deal could max out at $22 million after incentives.
Williams, a third-round pick in 2013, caught 44 passes for 594 yards and four touchdowns last season. His best asset is his availability -- he has never missed a game or practice in his career.
In his four seasons with the Cowboys, Williams caught 177 passes for 2,791 yards and 20 touchdowns and had two 100-yard games. He posted career highs of 52 catches for 840 yards in 2015 with Dez Bryant missing seven games to injuries.
ESPN's Todd Archer and Adam Caplan contributed to this report.