And the Guest Kardashian in Ariana Grande's 'Thank U, Next' Video Is ...

When Ariana Grande recruited a Kardashian to guest star in her highly anticipated "Thank U, Next" music video, she went straight to the top ... of the family tree.

Yes, TMZ has learned Kris Jenner is the mystery Kardashian who scored the cameo role. Take that, Kim, Kourtney, Khloe, Kylie, Kendall ... and Rob.

As we first reported ... Ariana's music vid -- which is an homage to several famous chick flicks -- drops on Friday, and the big question was which Kardashian she'd lined up to make their dancing debut.

Ariana posted a behind-the-scenes teaser of the making of the music vid -- check it out for some of the other celeb cameos.

We admit it, the odds-on fave had to be one of the sisters who frequently flaunt what they've got. Just remember, they all got it from their mama, which could explain why Kris landed the gig.

"Thank U, Next" has a ton of guests helping AG set up flicks like "13 Going on 30," "Mean Girls," "Legally Blonde" and "Bring it On."

We don't know exactly which movie Kris is getting dropped into ... but our best guess is Amy Poehler's "Cool Mom" from "Mean Girls."