At the Cannes Film Festival last month, it was pure madness. One night, Sharon Stone is on a rich Saudi’s yacht alongside an ostrich. Another night, a shirtless Justin Bieber is serenading a confused looking Busta Rhymes with his own breathy rendition of “Happy Birthday.”

And there, chillin’ beside the couture-clad celebrities, pouring bottles of rosé down his belly, just because, is “The Fat Jew,” a pudgy, bearded New Yorker.

Wait, who?

The Fat Jew (birth name: Josh Ostrovsky) is a 30-year-old comic Instagram star with nearly half a million followers.

Ever since a video of him teaching homeless people “SoulCycle” classes on parked Citi Bikes last summer went viral, the Chelsea resident, who got kicked out of Skidmore, dropped out of NYU and graduated from SUNY Albany, has taken his cult following mainstream.

The 6-foot-2, 250-pounder just sold two scripted shows to Amazon and Comedy Central — he’ll write and star in both. He’s got a book deal, and, oh yeah, Katy Perry and Snoop Dogg are fans.

“Some people say, do one thing and do it well. I say, do many things and do them very mediocrely,” quips Ostrovsky, who will be paid $1,000 to judge a pregnant women’s wet T-shirt contest in Tampa, Fla., next month.

His other mantra? Maintain an air of mystery. “I go 10,000 different names so people are consistently confused what to call me,” The Fat Jew explains. “Fabrizio Goldstein, Joshua Onassis, Jewson Surandon, or Fatrick Jewing…In the bedroom, I like to go by Bruce,” he adds.

Ostrovsky, who makes up to $2,500 for a sponsored Instagram post with one of his signature outlandish captions, is churning out one liners at his favorite West Village gay bar. He’s not gay — he and his fiancée, publicist Katie Sturino, share a three-bedroom apartment in Chelsea — but he loves the sliders.

His stomach protrudes as he sways to “Moon River” on the jukebox. He’s sporting his “hair erection” — a solid, vertical 10-inch ponytail — and a denim vest, sans shirt, that says “Mazel Tuff” on the back.

“That’s my motorcycle gang,” Ostrovsky says. “We’re nine Jews . . . weekend warriors, orthopedic surgeons, and on the weekends they’re like, ‘Let’s ride.’

“We’re not going to pop wheelies and get into bar fights. So we ride to Westchester on a nice day and get prosciutto paninis and use hand signals and go the speed limit. Prosciutto is about as badass as we get.”

Well, almost as badass.

Ostrovsky’s comedy skews toward the inappropriate. Last year, he had a Web series called “Hookers Doing Nonsexual Stuff,” where he invited prostitutes over to do things like cook him pasta and re-enact “Braveheart.”

In October, hundreds of fans protested outside Instagram’s NYC offices when he was kicked off the social media site for the third time for posting “explicit content.”

“He’s a cultural icon,” says NYC comedian Scott Rogowsky. “Funny just naturally emanates from his sweat glands. It’s his musk.”

“There’s this sense of discovery with him, like, ‘Oh my God, you’re real.’ People come up to him and just grab his hair,” says friend and entertainment reporter Ben Lyons, who says that a Cannes street was closed when a group of 12 crying French teenagers rushed Ostrovsky.

And to think that TFJ’s just a nice Jewish boy who grew up with his radiologist father, nutritionist mother and a younger brother who’s now an amateur body builder who works at the Pentagon.

Ostrovsky attended the tony Trevor Day School in the UWS, bouncing around a few universities afterwards before settling in at Suny Albany: “I was only hanging out with Italian dudes who had never met a Jew before.”

“I remember one time eating Boar’s Head and they were like, ‘BRO!!’ and threw it out the window,” says Ostrovsky, who owns 20 pairs of Stubbs & Wootton $500 slippers and schleps his two Cavalier King Charles Spaniels around like a pair of rare Birkin bags.

“They brought their own cold cuts from Staten Island.”

Growing up, Ostrovsky’s goals were less bottles with Busta more marmalades with Martha.

“I was super gay as a youth,” says Ostrovsky. “I wanted a house on Shelter Island and a partner and I wanted to go into event planning and do tasteful floral arrangements.” His March bar mitzvah theme was “autumn.”

The Fat Jew says his alter ego was birthed at summer camp. “It came out of a counselor I had who was super fat and identified himself as ‘a fat Jew.’” While Ostrovsky dabbled in “performance art” in college, he started making a living doing it after returning to NYC post-graduation.

He got paid to host parties as TFJ and film himself acting ridiculous. When he joined Instagram in 2012, the fans flocked like never before. He became the Famous Fat Jew.

But maybe not fat enough.

Recently, he says, “This little Puerto Rican girl ran up to me on Houston Street and was like, ‘Yo, honestly’ — she could’ve been 11 or 25, I have no idea — She was like, ‘Honestly, you’re not as fat as I thought you were. You’re f–king up. You need to go right now and get fat.’”

“I’m really trying to go for that young Gandolfini look, where I’m fat but taut,” explains Ostrovsky. “But I might just go right now and start drinking Nutella.”