Cliff Alexander was such a star H.S. basketball recruit ... one cop remembered him during the former NBA player's gun arrest this month -- and the interaction was all caught on police video obtained by TMZ Sports.
Alexander was pulled over at around 9:30 PM CT on May 6 in Chicago, Ill. after cops say he blew through a stop sign ... and shortly after, one officer realized who exactly he had detained.
"Oh, you're that hooper!" the cop said, before asking him about his high school recruiting ranking.
FYI -- Alexander was largely considered the top prospect in the country back in the 2014 recruiting class .