He replaces Tim Lewis, who was fired by Giants head coach Tom Coughlin at the end of the regular season.
Spagnuolo has been with the Eagles since 1999, first as a defensive quality control assistant. In 2001, he started coaching the team's safeties and eventually took over the entire defensive backfield.
He became the linebackers coach three seasons ago, working for Eagles defensive coordinator Jim Johnson.
Spagnuolo's coaching career includes 15 seasons as a college assistant as well.
As a unit, the Giants ranked 25th in the NFL in yardage allowed
(342.4 yards) and 28th against the pass (228.1 yards). They were
worst in the league when opponents got inside the 20-yard line,
allowing them to score touchdowns on 30 of 46 chances in the red
zone, or 65.2 percent.
Defensive shortcomings were a prime factor in the Giants losing
four games in the second half of the season in which they led or
were tied in the fourth quarter. The capper came in a 23-20 loss to
the Eagles in the wild-card game when the Giants tied the game with
5:04 to play, but allowed the winning drive as time expired.
The Associated Press and ESPN's Sal Paolantonio contributed to this report.