Chicago PD sources tell us ... the actor, who's most famous for playing Reg on "The Chi," was the victim of an armed robbery early Sunday morning in Chicago.

We're told Barton was a passenger in a car when someone slid into the back seat out of nowhere ... brandishing a handgun. Our sources say the robber demanded the car, but he ended up bailing on the plan.

Instead, we're told the assailant fled the scene with some of Barton's belongings ... a chain was taken, and possibly other items as well.

Thankfully, no injuries were reported ... but no arrests have been made. Cops are on the hunt.