The team officially announced the move later on Saturday.
"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," Saints coach Sean Payton said in a press release.
It's been an odd offseason for the former Dallas Cowboys coordinator. He was unceremoniously fired in Dallas, then hired/not hired in St. Louis. Now he lands in New Orleans, which might be the best spot of the three. Ryan won't have to win games with his defense, since Drew Brees runs one of the most dangerous offenses in the NFL. The 2012 season for the Saints broke a three-year run of playoff berths, and was more of a fluke as a result of of the bounty suspensions.
"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," Ryan said. "I have a ton of respect for (owner Tom) Benson, (general manager) Mikey (Loomis) and the organization and 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."
Payton is back and wants Ryan to install his 3-4 scheme. The changeover shouldn't be that difficult, considering they didn't play the 4-3 all that well. The defense only has to be average for the Saints to become one of the league's top teams again. They won seven games with the worst total defense in 2012.
The fired Steve Spagnuolo ran some 3-4 looks, but Ryan will need a few new bodies to fit the system full-time -- a pass rusher, cornerback and safety are first on the list.
Follow Kareem Copeland on Twitter @kareemcopeland.