Hope Davis didn’t realize the hard truth until her pals uploaded a batch of unflattering photos to Facebook and Instagram — she had resting bitch face.

“I was like, ‘Oh great, I look mad in the middle of the party,’ ” says the 42-year-old Red Bank, NJ, resident and ex-New Yorker of 17 years. “I looked like a sourpuss.”

Davis, whose job as a medical equipment sales associate depends on her appearing approachable, turned to a plastic surgeon.

“This is actually a common request from patients — I get several each week,” says Dr. David Shafer, a double board-certified plastic surgeon and medical director of Shafer Plastic Surgery & Laser Center in Midtown.

“They may not always use the words ‘resting bitch face,’ but if I mention ‘RBF,’ they say, ‘exactly.’”

Davis told Shafer she “didn’t want a ‘Joker’ smile,” but a “pleasant resting look.”

To achieve the look, doctors use techniques such as the injection of fillers into the face and sometimes Botox, medical experts said. The procedure takes about 10 to 20 minutes, costs between $500-5,000, with top docs, depending on the number of shots. It typically lasts up to two years.

Although the term RBF entered the cultural lexicon about six years ago, requests for the procedure “more than doubled” over the last year, says Shafer.

It’s because of a public shift in focus from the upper to lower face — “popularized by the Kardashians,” he says, and their affinity for lip injections.

He says selfies are also a factor: They force people to “look down at their phone, [which] accentuates the resting bitch face.”

A tiny bit of bruising and swelling can occur near the mouth and lips, but “it’s barely noticeable,” says Davis, who had the procedure done Sept. 4.

“Nobody can quite put their finger on it, but they notice something’s different,” says Davis. “People have definitely complimented me saying, ‘Oh you look so pretty and cute today.’ ”

That makes sense to Park Avenue plastic surgeon Dr. Melissa Doft.

‘The worse the ‘bitch face,’ the more effective the Botox.’

“People gravitate to women who they perceive as happy,” she says.

Doft said she injects fillers into their marionette lines — from the lip’s corners to the jawline — and underneath their lips to plump and re-angle their mouths.

The needles contain hyaluronic acid dermal fillers such as Juvéderm Vollure, which combines with water and expands into a gel that helps replace lost volume, replace lost volume contour the soft tissue and support the lips.

It helps “make [patients] look less sad” she said.

Every specialist has a different technique, but Shafer also likes to add Botox to the jowl area, called the DAO muscle, to freeze the frown in its tracks.

“The worse the ‘bitch face,’ the more effective the Botox,” says Shafer. “If you always look dumpy, or unfriendly … people are going to react to you differently.”

Two weeks after getting pumped, Davis thinks it’s working.

But there was one very unexpected side effect.

“I caught a glimpse of myself out of the corner of my eye, and it gave me a positive vibe because I looked happy,” says Davis. “This whole time, [I was focused on] how I project to the world, but I wasn’t paying attention to how I project to myself.”