The rookie phenom is ringing in his 20th birthday by channeling his favorite comic book supervillain, Thanos ... copping the Marvel-themed pendant -- with a minor twist.

Remember, Zion has gone on record saying his fav character is the bad guy from "Avengers: Endgame" ... and even said he wanted Thanos to BEAT Captain America and the gang at the end of the movie.

Spoiler alert -- Thanos didn't win ... he actually dies when Iron Man gets a hold of the gauntlet and snaps his fingers, thus killing himself in the process (RIP).

But, that didn't stop Williamson from staying loyal to his man ... with celeb jeweler ZoFrost gifting the young baller his own blinged-out gauntlet, which is fittingly holding a basketball.

The piece is incredibly detailed ... and even features all the Infinity Stones!!!

Of course, the Association is resuming its season at the end of the month ... and Zion's Pelicans are gearing up to make a last-minute playoff push.