Former NBA player Cliff Alexander -- the top H.S. recruit in the country in 2014 -- was arrested Wednesday ... after cops say they found a loaded gun on him during a traffic stop.
TMZ Sports has learned ... Cliff was pulled over at around 9:30 PM CT in Chicago, Ill. after cops claim he failed to stop at a stop sign at an intersection.
Officers say when they questioned Cliff ... he admitted to having a firearm under his driver's seat -- and when officers checked, they say he did indeed have a loaded pistol in the car.