The agreement will include a team option for a fifth year.
The team also came to terms with the other seven members of its 2012 draft class, an unusual move given that a team's draft picks are usually signed over time in the weeks following the draft.
The terms of the deals announced Monday were not released.
Irvin, chosen 15th overall from West Virginia, becomes the initial 2012 first-round choice to reach agreement. ProFootballTalk.com initially reported the agreement.
Irvin was the first outside pass-rusher selected in the 2012 draft and selecting him so high was considered a reach by some. He had been considered a likely second-day pick, but he became the centerpiece of a draft focused on defense, with most of Seattle's selections landing on that side of the ball.
The Seahawks also made improving their pass rush a top priority this offseason, signing tackle Jason Jones in free agency and drafting Irvin to assist Chris Clemons, who had 11 of their 33 sacks last season. No other Seattle player had more than four sacks in 2011.
The team envisions using Irvin with Clemons on passing downs. Irvin is a candidate to succeed Clemons in the "Leo" role eventually.
Information from The Associated Press contributed to this report.