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. They later located Nelson and threw him behind bars.
Court records show Nelson was charged with one count of carrying a loaded firearm on/in person vehicle ... and was arraigned on Dec. 26. He's now due back in court in late February for another hearing on the matter.
Nelson Jr., a defensive back, played for ASU from 2011 to 2013 and became a star for the Sun Devils in his senior year, recording First-Team Pac-12 honors after logging six interceptions.
He went unpicked in the 2014 NFL Draft ... but still managed to latch on with several NFL teams -- earning playing time with the Browns, Texans and Jets from 2014 to 2017.
The 33-year-old most recently played for the Arlington Renegades in the XFL in 2023.