It was a 30-degree December day, but to the 25 bystanders shoving their iPhones 3 inches away from Jen Selter’s butt, it was hot hot hot.

Selter, a 20-year-old Instagram star, famed for her large derriere, was demonstrating her squatting technique for the New York Post atop a subway railing in Midtown, clad in nothing more than a sports bra and hotpink yoga pants.

“Is that real?” one guy panted. Another asked for her hand in marriage. Three Hasidic Jews looked on in awe. The police came. And then stood there and enjoyed the show.

Selter didn’t even flinch.

“I’m recognized wherever I am,” says the 5-foot-6, 112-pound Selter, who, since joining Instagram in March 2012, has amassed more 1.3 million followers, including Rihanna, football player Terrell Owens and basketball star Amar’e Stoudemire. “I don’t really go to public gyms anymore just because it is a whole big scene when I’m there,” says Selter. “I don’t like being watched.”

But she doesn’t mind being “liked.”

A seductive post of hers on the photosharing site — most of which are selfies of Selter in skintight gym wear — can easily rack up more than 70,000 likes.

“I don’t really post a lot of face pictures,” admits Selter, whose handle is @jenselter.

“I mainly do body selfies. Not that I care what people think, but they don’t care. They don’t want to see my face,” she says.

Selter, who skipped college for Internet stardom, won’t disclose how much she’s made on her fanny fame thus far. But deals with NYC water company NY20 and nutrition supplement company Game Plan Nutrition, for which she is a spokeswoman, have helped her rake in “a lot more money than a graduate would be making,” according to Selter.

She plans to further capitalize on her success with her own line of workout wear — “Everyone always wants to know what leggings I’m wearing. So why not make my own?” — and maybe a chain of “like cool gyms, like with some glowing stuff,” she says, imagining a paradise akin to David Barton gyms.

“I see myself motivating and inspiring everyone around the world,” she explains, maintaining that her photos — like the close-up shot of her sitting atop a workout ball — will remain G-rated.

“I’ll never post a raunchy pic. There’s a difference between a porn-site picture and gym wear or bikini wear. Everything’s usually yoga pants,” says Selter, who, despite her sexed-up image, is a surprisingly naive and soft-spoken girl who uses the word “dope” and lives with her mother on the Upper West Side. She owns more than 150 pairs of yoga pants.

“Yes, it can be showy, but I’ve seen a lot worse. And if it motivates people to get their butts up and go to the gym, why not?”

The Roslyn, LI, native has gained a cult following for her unique ability to simultaneously serve as inspiration for tens of thousands of women and a sex symbol for hundreds of thousands more men.

“My initial impression was sort of like a fascination,” muses 28-year-old NYC fashion photographer Ben Fink Shapiro, who started following Selter on Instagram three months ago.

“It’s sort of like a silent, miraculous observation. Like, how is a body shaped like this?” asks Shapiro, who says his girlfriend is “equally as fascinated and obsessed with her.”

“She’s so skinny. She doesn’t have any hips, and then, BOOM, this ass comes out of nowhere, like, God damn!”

To Selter, the fame was a sneak attack, too.

After graduating from high school, she took a smattering of cosmetology classes while holding side gigs at a plastic surgeon’s office and at a gym at the Roosevelt Field Mall.

Selter started working out — and her butt began ballooning.

So she did what any millennial would do: She took some belfies (that’s butt selfies, for all the ancients out there).

“It’s cool seeing your body transform. That’s what motivates me,” says Selter, who stresses that “fitness isn’t just about looks. It’s about how you feel.”

She started posting workout selfies on Instagram in March 2012. She noticed people were reposting her images without credit, so she reached out and asked to be tagged. Before Selter knew it, her fan base got as plump as her rear, amassing nearly 200,000 followers every month. (She quit her gym job when she hit 500,000.)

“I’m very proud of her because this is a girl who didn’t want to go to college, and she was able to build up this social media in such a way that she has become famous and she is an inspiration and motivation for so many people,” says Selter’s mother, Jill Weinstein, who only asks that her daughter doesn’t end up in Playboy.

“We can’t even get down the street without people stopping us and saying, ‘Oh my God, is that Jen Selter? Can we take a picture?’ ”

But with fame came the haters. Selter’s been accused of having a surgically enhanced booty and has been called everything from a “butterface” to, well, things that would make porn maven Jenna Jameson blush.

Selter doesn’t let the negativity bum her out.

“I’ve seen fake butts, and they’re very nice,” says Selter. “They look nothing like mine. I see it as a compliment.”

“My body’s all real . . . from boobs down,” adds Selter, who was profiled about her high school nose job in an April 2010 People magazine article about teenagers going under the knife.

“I grew up in Long Island. I’m Jewish. Jewish girls have big noses,” she says. “Probably 1 out of 3 girls I know [has] a nose job.”

A single Selter is keen on keeping her head on her shoulders and her butt (and dreams) as high as the sky.

“I didn’t sign up to be a model or some Instagram person who everyone is judging and looking at,” says Selter.

“With hard work and dedication, anyone can get to where I am.”

Selter’s Ass-pirational Advice

“Your butt is your biggest muscle. There are so many ways to shape it and firm it up,” says Selter. Want to back it up like Selter? Try some of her go-to moves.