Williams, a six-time Pro Bowl player and former defensive captain with the Minnesota Vikings, will make in excess of $2 million, according to league sources.
Williams will be entering his 12th season in the NFL, having spent his entire career until now with the Vikings. Williams started 15 games last season for the Vikings with 3½ sacks and an interception.
Seattle found success last season with a deep rotation along the defensive line, an area that took a hit during free agency following the Seahawks' Super Bowl title.
Seattle wrapped up its organized team activities on Thursday. The Seahawks still have their mandatory three-day minicamp beginning Tuesday.
Information from The Associated Press was used in this report.