He joins general manager Dave Caldwell, who led the coaching search after being hired last week.
"It was just a matter of time before Gus Bradley became a head coach in the NFL, and the Jacksonville Jaguars are extremely fortunate that Gus will be on our sidelines for many years to come," Caldwell said in a statement. "Gus more than met every criteria we insisted on from our new head coach, and his intangibles and leadership abilities are exceptional. Gus is who the Jaguars need now and in the future."
Bradley spent the last four seasons in Seattle, where his defense improved each of the last three years and finished in the top 10 in points and yards the last two. This season, the Seahawks ranked first in the NFL in points allowed (15.3), fourth in yards (306.2) and tied for fourth in takeaways (31).
The Jaguars were 30th in the league in total defense in 2012.
Owner "Shad Khan and Dave Caldwell expect to win, and that's what I wanted to hear," Bradley said. "That's why I am coming to Jacksonville to win a Super Bowl."
Bradley will be introduced at a news conference Friday.
He began his NFL coaching career with Tampa Bay as a defensive quality control coach in 2006. He was the Buccaneers' linebackers coach the next two seasons before going to Seattle. Bradley coached in college from 1990-2005, including two stints at his alma mater, North Dakota State, and four years at Fort Lewis College (1992-95).
But his rise through the NFL ranks had him on several teams' radar. He also interviewed for the head job in Philadelphia this week.
"He's got a brilliant football mind," Seahawks coach Peter Carroll said this week. "He's got a way of reaching people and touching people and getting the best out of them, coaches and players alike. He's got everything that you're looking for."
The Jaguars interviewed defensive coordinator Mel Tucker, St. Louis Rams offensive coordinator Brian Schottenheimer and Cincinnati Bengals offensive coordinator Jay Gruden before striking a deal with Bradley.
Bradley replaces Mike Mularkey, who went 2-14 in his only season in Jacksonville. Mularkey failed to make the team any better in his first season.
Khan fired general manager Gene Smith, the architect of the roster the last four years, and charged Caldwell with turning around one of the league's worst franchises. Caldwell's first move was ousting Mularkey, saying the team "needed a fresh start."
"I'm looking for a co-builder of our team," Caldwell said last week. "I felt like it was an atmosphere of change. I felt like that to do that, you've got to have a fresh start across the board."
Many believed Caldwell would target close friend and college roommate Greg Roman, San Francisco's offensive coordinator.
Instead, Caldwell and Bradley will team up in hope of getting the Jaguars back to the playoffs for the first time since 2007. Jacksonville has missed the postseason 11 times in the last 13 years.
"The relationship between the general manager and the coach is vital," Khan said last week. "It has to be a symbiotic relationship and they have to grow together and the coach has to be somebody that it's very, very important to win and very, very important for Jacksonville."
Bradley inherits a team with few playmakers on either side of the ball.
The Jaguars have running back Maurice Jones-Drew under contract for another year and have young and talented receivers Justin Blackmon and Cecil Shorts III. But the offensive line was a mess in 2012, adding to the team's quarterback woes.
Neither Blaine Gabbert nor Chad Henne proved to be the answer.
Caldwell said he had "others in mind" to compete for the starting job.
Defensively, the Jaguars could lose linebacker Daryl Smith, defensive tackle Terrance Knighton and cornerbacks Derek Cox and Rashean Mathis to free agency. The more pressing issue will be how to generate more consistent pass rush.
The Jaguars had a league-low 20 sacks this season. Philadelphia Eagles cast-off Jason Babin helped down the stretch, but the Jaguars are likely to use the No. 2 pick in April's NFL draft to find a pass rusher.
Bradley helped develop rookie defensive end Bruce Irvin this season. Irvin, the 15th overall pick, led all rookies with eight sacks. His defense had other young stars, too.
Linebacker Bobby Wagner, a second-round draft pick, ranked second among rookies in tackles with 140 and fourth with three interceptions. Safety Earl Thomas was voted to his second consecutive Pro Bowl. Second-year cornerback Richard Sherman led the team with eight interceptions, and defensive end Chris Clemons has a career-high 11 1/2 sacks.