Doja Cat had fans puckering up in Paris ... but they weren't there to kiss her lips ... they were kissing her ass ... literally.

Check out this cheeky clip ... the singer was at the MAC x Sephora Champs-Élysées launch Monday when she turned around to pose for photos ... giving fans the perfect opportunity to bend down and plant kisses on her backside while snapping selfies.

Doja, who showed up in one of her signature bold and quirky looks, ended up covered in lipstick marks ... turning herself into a walking billboard for the beauty bash.

Before anyone clutches their pearls, this wasn't some case of fans getting too handsy. The event was a promo for MAC and Sephora, and the lipstick-covered kisses appeared to be part of the activation.

Bottom line ... Doja's fans were up her ass ... in the best way!!!