Rihanna went to support A$AP Rocky's fashion pursuits ... but her outfit gained the most attention when she accidentally flashed her backside.

The singer-songwriter stepped out in an all-black ensemble Friday night to attend her man's AWGE fashion show in New York City ... and her leather jacket opened a bit too wide and revealed the star's famous derrière.

Check it out ... the jacket's skirt is split in the back and the split reaches just a bit too high up her legs, revealing the bottom of her bottom.

RiRi quickly readjusted the jacket ... stopping the crack from showing any of hers.

Despite the fashion faux pas, Rihanna looked chic as always ... just like she did recently at Paris Fashion Week, where she flashed her cleavage and even during a recent trip to the grocery store.

Take a bow for your fashion, Rihanna ... but maybe grab a different skirt first!