The NFC South side fired Steve Spagnuolo after one season in the post at the end of the 2012 campaign, after the side set a new record for most yards allowed by a defence in a single season.
Head coach Sean Payton said in a statement: "In regards to Rob, we have experience in preparing and playing against his defenses and they've always been challenging in terms of the different looks and pressures that they feature."
Ryan added: "I have had the opportunity to get to know Sean Payton and his staff a bit better recently and I am excited about joining the team. I'm ready to get to work on all facets of the game. This is a great opportunity and we're getting started right away."
Ryan, who was fired by the Dallas Cowboys last month, had been linked to a role with the St Louis Rams, but talks between the two parties fell through.