The Broncos made the announcement via Twitter on Friday night. They needed to replace Dennis Allen, who spent last season as defensive coordinator but left to become head coach of the Oakland Raiders.
"We are thrilled to be able to add such a well-respected defensive coach to our staff," Fox said, per the Broncos' Twitter account.
"He was an integral part of our turnaround on defense during my first year with the Panthers in 2002. His defenses with the Jaguars have ranked among the NFL's best, and we are excited he's a Bronco."
Del Rio was fired by the Jaguars in November on the same day the team was sold to businessman Shahid Khan.
The Jaguars were 3-8 when Del Rio was fired in the midst of his ninth season as head coach. He went 68-71 at the helm in Jacksonville but the team failed to post a winning record in each of his last four seasons.
Jacksonville had just three winning seasons under Del Rio and reached the playoffs as a wild card in 2005 and 2007, posting a 1-2 postseason mark.
Under Allen, the Broncos went from having the worst defense in 2010-11 to having the 20th-ranked defense this past season.