Zion Williamson Declares for 2019 NBA Draft Because Duh

File Under, "The Most Obvious News Ever" ... Zion Williamson is leaving Duke and declaring for the NBA Draft.

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.