'Butt Day!' Kate Upton Bares Her Entire Backside in Sheer Dress

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When you look like Kate Upton, underwear is optional.

The 23-year-old supermodel
shared a booty-baring photo, posing in a see-through, flowing burgundy dress. "Happy Sunday! #buttday #bruceweber #sundayfunday," she wrote.

The cheeky pic isn't the first of its kind this summer. In fact, so many celebrities are baring their bums on Instagram, it's starting to become a trend.

WATCH: Kate Upton Reveals Her Supermodel Diet -- 48-Hour Juice Cleanses and 'Tequila Tuesday'

Justin Bieber flashed his butt earlier this month, only to take it down after deciding that it might offend his younger fans.

Chrissy Teigen also showed off her supermodel butt in a thong bikini.

Prior to that nearly-nude image, she posted a pic of her husband John Legend's backside. Only tagging @Instagram, she kept the photo captionless, letting Legend's butt do all the talking.

WATCH: Khloe Kardashian Shares Unretouched Booty Photo to Shut the Haters Up

While Upton and these stars willingly shared their butt selfies with the world, the supermodel recently admitted that she was "horrified" when a video of her seductively dancing in her teeny bikini was released by photographer Terry Richardson in 2012.

"I was like, 'That was disrespectful, you could have told me!" she recalled to Vogue U.K. "Now, obviously, it's fine."

See why Upton was so upset about her "Cat Daddy" dance going viral:

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