The deal is for two years and $15 million. Avril turned down a three-year, $30 million deal last season. Avril was considered by some as the top free agent in the market.
Avril, who played with the Detroit Lions last season, has 29 sacks, and nine forced fumbles over the last three seasons. In 2012, Avril had 28 tackles and 9.5 sacks.
Avril was given the non-exclusive franchise tag by the Lions, paying him $10.6 million last season.
The Seahawks upgraded their offensive talent by trading for former Minnesota Vikings receiver Percy Harvin earlier this week. The team then signed him to a six-year, $67 million contract, with $25.5 million guaranteed.