On a recent Wednesday night, about 70 fitness devotees wearing in-the-know brands like Heroine Sport and Outdoor Voices were crammed into a studio in the Flatiron district, mastering their leg lifts while Pink Floyd’s “Comfortably Numb” blared overhead. Taryn Toomey, a founder of the workout program called “the Class,” led the group.

But this was not another fitness upstart in a body-obsessed neighborhood dotted with dozens of boutique studios and several major athletic clothing brand stores.

It was the new flagship store of Bandier, a two-year-old retail brand that specializes in high-end activewear for women seeking $400 yoga pants and other fashionable alternatives to giants like Lululemon and Athleta.

The boutique, which started in Southampton, N.Y., is the creation of Jennifer Bandier, a 47-year-old former R&B manager, vintage clothing peddler and fitness enthusiast, who comes across like a sweet and hyperactive cheerleader.

During an afternoon visit at the store, she brought up such seemingly random topics as the Tennessee origins of her rescue dog, Dixie; her desire to flip houses after watching HGTV; and how she came to start a fitness retail chain with next to no sales experience.

“I’m the type of person who is either at a one or 3,000,” said Ms. Bandier, wearing wide-legged jeans, a pinstriped silk button-down and navy suede boots.

The concept for the store was born out of her own frustrations in 2013, when recovery from a broken foot had Ms. Bandier largely sidelined and living in workout clothes. Wanting her clothes to be as stylish as the Frame Denim and Isabel Marant she prefers, she scoured the web and found smaller brands, like Toronto-based Michi and Los Angeles-based Splits59. But she couldn’t find a boutique to try them on.

So in the summer of 2014, Ms. Bandier opened a 1,200-square-foot concept store on Main Street in Southampton that sold high-end fitness wear like a Michi “Feline” sports bra ($129) and technical fabric leggings from a London label, HPE ($125).

The store was a hit. At a time when Lululemon seemed like the only name in the game, the beach-body-conscious women of the Hamptons were drawn to the store’s well-edited selection of fashionable workout brands.

“If it doesn’t work in Southampton, where on earth would it work?” said Jayne Harkness, Bandier’s chief merchant. “Everybody who walked in the store looked around and took a minute to digest what it was, and then they engaged immediately.”

By November of that year, Bandier opened its store in Manhattan, a 2,300-square-foot-space at the corner of 21st and Broadway. “Once we saw it was going so well in Southampton, we quickly shifted gears,” said Neil Boyarsky, Ms. Bandier’s husband and the company’s chief executive.

Ms. Bandier’s career path is a colorful one. The daughter of Martin Bandier, the chief executive at Sony/ATV Music Publishing, she was once a manager of the hip-hop group TLC with her husband at the time.

Interactive Feature | NYT Living Newsletter Get lifestyle news from the Style, Travel and Food sections, from the latest trends to news you can use.

Next, she started Luxorama, a company that sold bags customized with customers’ photos on QVC, and was briefly an owner of JBM Vintage, a by-appointment-only vintage clothing store on East 75th Street, until she realized she was more interested in collecting vintage clothes than selling them.

“It took me a long time to find my way,” Ms. Bandier said. “Now that I have, I’m having so much fun.”

When the lease for the Broadway space came up, she moved the store to a 3,000-square-foot storefront on Fifth Avenue and West 21st Street. Ms. Bandier wants shoppers to stay and hang out, so there’s a seating area up front. The fitness studio is up a staircase covered in spray-painted hearts.

Bandier also has locations in Southampton and the Upper East Side, as well as at luxury shopping centers in Manhasset, N.Y., and Dallas. Additional stores in New York, California and Florida are planned for this year.

Established companies have taken notice of Bandier. Reebok, for example, has introduced certain products exclusively at Bandier. “We really believe they are trailblazing and have kind of shaken up the world” of fitness wear, said Catherine Marshall, director of global business development at Reebok. “They have a hyper-engaged audience.”