The Olympic gold medalist floored his opponent three times in the fourth round, forcing the referee to call time on the fight.
DeGale, who recently turned down a super-middleweight title shot against Robert Stieglitz, controlled the fight from the off, warding off Sanavia's blows in the first round, and landing a few of his own.
The Italian landed a powerful left hook in the second round, but DeGale responded well with some deft uppercuts, and went on to control the third.
DeGale pulled out all of the stops in the fourth round, putting his opponent down three times in quick succession before killing off the encounter with a well-placed body blow.
It was DeGale's first title defence since he claimed the crown with a majority decision over Piotr Wilczewski in October 2011.