A person familiar with the decision told The Associated Press on Thursday about the hiring on condition of anonymity because the Saints have not announced it.
Before taking over in St. Louis, Spagnuolo had top 10 defenses with the Giants in the 2007 and 2008 seasons. His 2007 defense led the NFL with 53 sacks and the 2008 unit was sixth in the league with 42.
Before joining the Giants, Spagnuolo was a defensive assistant in Philadelphia from 1999-2006. He coached Pro Bowl players including Brian Dawkins and Lito Sheppard, and helped Philadelphia field a 10th-ranked defense in the 2004 season, when the Eagles made it to the Super Bowl before falling to New England.