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 ... right alongside future Duke Blue Devil Jahlil Okafor.
In the video, you can hear the officer actually asks Alexander about the battle for the No. 1 spot with Okafor ... and the interaction is kind of funny.
"I remember that sh*t!" the cop said. "That's crazy! ... You went to Kansas, right!?"
Of course, the stop didn't go so well for Alexander outside of that convo ... cops say the 24-year-old illegally had a loaded firearm under his seat, which one officer said appeared to be a "Glock 21."
Alexander also admitted to cops during the stop he had "like 6 grams" of weed on him ... and you can see in video, he explained to an officer he stuffed it in his pants because "I'm paranoid."
Eventually, Alexander was arrested and booked on a felony charge of aggravated unlawful use of a weapon. He was also hit with a citation for failing to stop at a stop sign -- but he was NOT booked on drug charges.
Alexander -- who spent just one season in the NBA after a disappointing first year at Kansas and now plays professionally overseas -- is due back in court in July for a hearing in the case.