Lil Yachty doesn't need a fancy hat for the Kentucky Derby ... he's got a new $90K chain for the occasion.

A rep for the rapper tells TMZ ... jeweler Erin Barnett hand-delivered the expensive ice -- a custom diamond chain and racehorse pendant -- at SoCal's Santa Anita racetrack on Derby Day.

Yachty was flashing the new bling at an owners-only party for Run Fast Racing app users on Saturday.

He's a partner of the Run Fast Facing app -- where fans can co-own racehorses and make money when they win. Yachty owns 16 horses himself.

And he's not the only celeb involved with the app ... Rauw Alejandro and Lil Wayne are also partners.

Lil Wayne's horse Sixhoofsevenhoof won first place at Los Alamitos Race Course last summer.