Bradley spent the last four seasons in Seattle. This season, the Seahawks ranked first in the NFL in fewest points allowed per game (15.3) and fourth in yards (306.2), and tied for fourth in takeaways (31).
The Jaguars were 30th in the league in total defense in 2012.
"He's got a brilliant football mind," Seahawks Coach Pete Carroll told the Associated Press. "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."
Bradley replaces Mike Mularkey, who went 2-14 in his only season in Jacksonville.