Spagnuolo spent the last three seasons as head coach in St. Louis, but was fired on January 2 after the Rams finished the 2011 season with a bleak 2-14 record.
He finished his brief tenure with a mark of 10-38. The Rams were 1-15 in his first year, then took a big step forward at 7-9 in 2010 before regressing this past season.
His Rams team was expected to contend in the NFC West in 2011, but started with six straight losses and never recovered. The Rams' only wins came against New Orleans and Cleveland in a three-week span. They finished the year on a seven-game losing streak.
He will replace Gregg Williams, who left New Orleans on January 16 to become defensive coordinator of the Rams under new head coach Jeff Fisher.
Prior to becoming the Rams' head coach, Spagnuolo spent two seasons as defensive coordinator for the New York Giants, calling the defensive plays for a New York squad that won Super Bowl XKII after the 2007 season against the previously-undefeated New England Patriots.