The Flatiron District has a new nickname in the running.

The neighborhood that surrounds the namesake, triangular landmark has quickly become known as the “Fitness District,” thanks to a plethora of boutique fitness studios and retailers where you can stock up on trendy leggings and tanks.

That’s not the only fitness-related moniker it’s inspired.

“We call it the ‘Fitiron,’” said Jennifer Bandier, who opened her flagship, three-story fitness retail store Bandier on Fifth Avenue between 21st and 22nd streets in January. “Flatiron is the center of boutique fitness and active lifestyle in NYC.”

There was a time when the neighborhood — whose boundaries are, somewhat generously, considered by the listings site StreetEasy to be from 14th to 30th streets and from Sixth to Lexington avenues — was better known as the Toy District, before being christened the Flatiron District by realtors in the 1980s.

Over the past few years, the studios have been moving in. In 2010, indoor cycling studio Flywheel opened its first location on West 21st Street between Fifth and Sixth avenues.

“There were some private training gyms around the neighborhood, but when we opened our Flywheel doors, we were the only boutique fitness business in the area,” said Flywheel co-founder Ruth Zukerman. “The block was mainly nightclubs and bars — the street would actually shut down on weekend nights to contend with the crowds!”

Today, the neighborhood’s transformation has followed the overall rise of high-end studios in the city, where classes can easily cost $34 or $38 a pop.

“You can’t walk half a block without passing a boutique fitness brand, gym or an athletic apparel retailer,” said Zukerman, who recently opened Flywheel’s eighth NYC location.

By its count, the Flatiron/23rd Street Partnership Business Improvement District found nearly 50 gyms and studios in and around Flatiron — including not one but two SoulCycles — with recent additions including yoga studio Y7 and boxing spot Shadowbox. A Barry’s Bootcamp is slated to open this fall in the area, too.

“There are more studios here than I think anywhere in the city,” said Daniel Glazer, founder of Shadowbox, which opened on West 20th Street between Fifth and Sixth avenues last May. “All the biggest brands and all the upstarts feel it’s not only a place they want to be, but have to be to be taken seriously.”

And if you build it, they will come. Within five blocks along Fifth Avenue, you can find activewear brands Sweaty Betty, Nike Running, New Balance and Tory Sport, the athleisure line of fashion label Tory Burch, which opened in March, as well as Athleta, Bandier and Lululemon — all of which have in-store studios for yoga and dance classes. Lululemon also has a concierge service that can help direct customers to nearby studios.

“You really feel the energy in the neighborhood,” said Julie Anderson, who helped create the Lululemon service. “It’s the most interesting and convenient neighborhood to get any kind of workout in. It really runs the gamut.”

The neighborhood’s transformation is owed to its central location in Manhattan, with access to multiple subway lines, as well as the rise in recent years of its residential and worker population.

“It makes sense to us, in the context of the overall growth of the neighborhood,” said Jennifer Brown, executive director of the Flatiron/23rd Street Partnership Business Improvement District, of the area’s fitness prowess. “So many more people live here now than they did five or 10 years ago. There’s also a lot of tech and creative companies.”

Between 2010 and 2015, the Flatiron population rose by more than 11,000, according to the Flatiron Partnership. By 2020, the neighborhood is expected to add another 10,000. More than 800 units are being added to the market in the neighborhood through new residential developments such as 10 Madison Square West, 45 East 22nd Street and 7 West 21st Street, according to the Flatiron Partnership.

In addition to residents, the neighborhood has a healthy mix of workers helping to sell out studio classes on their way to or from the office. According to the Flatiron Partnership, there are more than 750,000 daytime workers in the area, in industries such as media, publishing, advertising and tech (which has helped earn the neighborhood another nickname: “Silicon Alley”). Major commercial tenants drawn to the nabe include Tumblr and Sony, and SoundCloud, Dropbox and startup Grovo have all signed leases at Two Trees Management’s 50 W. 23rd St. building. Coworking is also big, with The Yard and three WeWork locations.

As the worker and residential population continues to grow, the Flatiron Partnership expects the fitness scene to keep up the pace.

“We have a pretty low vacancy rate right now ... but we imagine this will be something that continues to be really popular,” Brown said.

Shadowbox, for one, has doubled its class offerings — from 30 per week when it opened to about 60 now — to meet the demand.

“We’re trying to keep up,” said Glazer, who is opening a second Shadowbox in DUMBO this summer. “We can’t open our second location fast enough.”