The summer’s hottest outfit is a shapeless spotted shmatta that costs $70.

It’s “the Zara dress,” a black-and-white polka-dot, long-sleeved, ankle-length number that’s as ubiquitous as last winter’s Amazon coat and as sought-after as an Hermès Birkin bag. And if you want to nab one, good luck: It’s sold out both online and in several NYC shops.

So, what’s the appeal? After all, it’s cheap, not necessarily practical and — as one acquaintance put it on Facebook — “Laura Ingalls Wilder borderline fugly.”

The frock’s fans say its charm lies in its shapelessness.

“It is like [a] one-piece bathrobe,” actress and comedian Julia Finkelstein tells The Post. “I’m very drawn to anything that’s very drapey and smock-y.”

The 25-year-old Los Angeleno first spotted a woman wearing “the dress” a couple weeks ago. “She looked so chic and effortless,” Finkelstein says. After the stranger “reluctantly” admitted it was from Zara, Finkelstein went out and bought it off the rack.

“It’s easy-breezy and makes me feel like Jemima Kirke in ‘Girls,’ ” she says.

That gauzy insouciance is hard to come by, says Olivia Jackson, a marketing manager and fashionista who chronicles her outfits on the blog Clothes My Boyfriend Hates.

“I’d struggled to find a really floaty maxidress,” says the 24-year-old Londoner, adding that most of the options available had more defined waists. The Zara ensemble “was different [from] a lot of what had been around lately . . . The fit is huge, more oversize than I’d normally go for, but I loved that I could throw it on with anything and it’d look good.”

Wearers claim that it looks good on a variety of body types too.

“I’m a bigger woman who is normally a size [14 US],” says Shannon Flack, a 23-year-old customer-service agent based in London, where the dress first caused a sensation. Zara didn’t make the dress in her size, but when Flack saw her “curvy” friend rocking it, she bought it a size down. “It actually fit perfect and is extremely flattering.”

Plus, with its neutral colors, plain silhouette and puritanical lack of adornment, the black and white polka dot dress provides a blank canvas for her to style in her own unique way.

“You can have it casual [and] wear it baggy with a pair of sandals or trainers, [or] you can [put] a belt and necklace on and maybe even a pair of heels,” says Flack.

Of course, not everyone loves having the same look as everyone else. When Finkelstein found out there was an Instagram account, Hot 4 the Spot, devoted to sightings of the Zara frock, she thought, “Well, I can never wear this dress seriously again!” (She did recently don it for an Instagram video, however.)

Jackson took another route. Sick of seeing every girl on “the tube” with the same dress as her, she dyed the whole thing pink and replaced the buttons with plastic heart enclosures.

“I’ve fallen back in love with it,” she says.

Her boyfriend? Not so much.

“He described it as a hospital gown and a paper bag, and said it’s just huge,” says Jackson. “But that’s what I love about it.”

The Zara dress is currently sold out — but you can still shop similar looks at several price points.