Free-agent contracts cannot be signed until 4 p.m. ET on Tuesday, but teams and players were allowed to begin negotiating on Saturday.
According to Wilson, the deal is worth $18 million over three years, with $7 million in 2015, $5 million in 2016 and $6 million in 2017.
Williams, 30, started every game of the last two seasons for the Philadelphia Eagles. Last season, he intercepted two passes, deflected nine and recorded 59 tackles. Set to make $6.5 million in 2015, Williams was released by the Eagles last week.
The Tennessee Titans selected Williams in the seventh round of the 2008 NFL draft. After bouncing around the Titans' active roster, practice squad and waivers, the Baltimore Ravens signed him in November 2009. He played for Baltimore through the 2012 season before signing a three-year, $17 million deal with the Eagles in March 2013.
Williams chose the Seahawks over offers from the Ravens, Titans and New York Jets, according to The Baltimore Sun.
- Mike Fiammetta