Robert Nelson Jr. -- a former Arizona State Sun Devils football star who spent three seasons in the NFL -- was arrested in Los Angeles last month for allegedly illegally possessing a gun ... TMZ Sports has learned.
Law enforcement tells us the arrest happened in Burbank back on Dec. 22.
We're told Burbank Police officers were patrolling a local mall parking lot at around 9 PM when they observed Nelson's vehicle in a handicap spot without a placard.
Cops say when officers looked into the car, they noticed a loaded handgun sitting in plain view on the floorboard.