Cole Anthony -- last year's 15th overall pick -- is confessing his biggest rookie year screw up ... laying down wayyy too much cash on a chain prior to draft night.

TMZ Sports ran into the Orlando Magic star only hours before the 2021 NBA Draft ... and we asked the 2nd year hooper out of UNC how he treated himself after being picked in last year's 1st round.

"Before the draft, I bought myself a chain which I immediately regretted," the 21-year-old said. "It was a lot of money at the time ... at the time, it was a lot. I ain't going to lie to you."

FYI, Anthony signed a near-$16 million, 4-year deal with Orlando in the November draft (it was delayed over COVID-19).

Tonight, 60 more young fellas are about to strike it rich ... and Cole has some sound financial advice for the soon-to-be millionaires.