The Calgary native was set to enter the option year of his original contract this season.
Originally drafted in the first round (sixth overall) of the 2011 CFL draft, Iannuzzi has recorded 38 receptions for 418 yards and five touchdowns over 31 career appearances for the Lions.
"Marco is gifted receiver who works extremely hard," Lions head coach Mike Benevides said in a statement. "He's part of a solid core of young receivers that we have a lot of confidence in and I'm very pleased to have him with us moving forward."
The Harvard grad, who also sees time on special teams, had a career-high 99 receiving yards on seven receptions in a game versus Edmonton last season.