Yeah, no kidding.

Williamson was a monster in high school and became a global sensation during his 1 year at Duke -- where he was widely regarded as the #1 player in the country.

Now, he's projected to be the #1 pick in the NBA Draft, according to anyone who's ever watched basketball.

The 18-year-old made his announcement in a video -- in which he thanked everyone at Duke and coach Mike Krzyzewski for "the best year of my life."

Zion says he learned more about basketball in his 1 year at Duke than he ever thought possible -- but ultimately decided to leave to pursue his dream of becoming an NBA player.

"I don't think I can put it into words how special this year was," Zion said ... shouting out his teammates for the "brotherhood" he experienced.

As for Zion's bank account ... last year's #1 pick DeAndre Ayton signed a 4-year, $40 million contract -- so expect Zion's to be a little bigger than that.

Zion will also sign a MASSIVE shoe deal now that he's going pro -- with Nike brand legend Sonny Vaccaro telling TMZ Sports he wouldn't be shocked if Zion got a $100 MILLION deal.