Rihanna is putting one of her pals to work, work, work, work, work … having a woman come over and rub her famous backside down with whipped oil body cream from her own brand.

The singer-songwriter appeared in a quick clip posted to a pair of Fenty Instagram accounts … lying on her front in bright pink lingerie — when a feminine hand dips into a small cannister of Fenty Skin Butta Drop Whipped Oil Body Cream and goes to town on RiRi’s booty.

Check out the vid … Rihanna gets a pretty thorough rub on her exposed cheeks — and, the video even slows down to give Fenty fans a good look at the jiggle.

In the caption, Rihanna pushes the Butta Drop and the Lavish Lace collection lingerie she’s wearing … a 2-for-1 sales pitch.

Rihanna’s pushed her Fenty lingerie hard in recent weeks … regularly stripping down to near-nothing to impress prospective buyers.

BTW … fans are loving the marketing ploy in the comments, with some saying they’d give anything to be the person charged with rubbing Rihanna down.