Financial terms of the deal weren't immediately available.
Hawthorne, 26, is coming off a season in which he notched a career-high three interceptions to go with 115 tackles. He became a starter in 2009 after signing with Seattle as an undrafted free agent out of TCU in 2008.
Hawthorne has led the Seahawks in tackles for three straight seasons, notching at least 100 tackles each year.
He played primarily middle linebacker for Seattle but can play all three spots and will likely play outside linebacker for the Saints, who recently signed former Atlanta Falcon Curtis Lofton. Lofton could replace defensive captain Jonathan Vilma, who is coming off a knee injury and facing potential league punishment for his role in the team's pay-for-pain bounty system.
According to a source, Hawthorne had a two-year offer on the table from the Seahawks, but Seattle rescinded it Tuesday afternoon.
Hawthorne visited New Orleans in March, and was en route to a visit with Detroit when the Lions re-signed Stephen Tulloch to a five-year, $25.5 million deal while Hawthorne was still on the flight.
New Orleans lost to Hawthorne and the Seahawks in the wild-card round of the 2010 playoffs, and new Saints defensive coordinator Steve Spagnuolo -- previously the head coach in St. Louis -- played against Hawthorne's Seahawks twice a year as NFC West rivals.
Hawthorne has quietly gained a reputation for his versatility, hitting power and ability to stop the run.
Hawthorne, Tulloch and Lofton were considered the top middle linebackers at the onset of free agency, but the trio quickly found a softer free-agent market than they anticipated. Lofton's backloaded contract averages $5.5 million per season, and the total package can reach $33.5 million provided he reaches escalators in the last three years of the deal.