A spa on Billionaires’ Row has been accused of operating a seedy brothel, advertising “Sweet, sexy, hot Asian” women, according to a complaint filed by the city.

The 57 Star Beauty Spa, which abuts supertall 111 W. 57th St. — currently advertising a $98 million listing — has been the target of two undercover police stings since December.

“It was somewhat of a little-known secret,” said a former fourth-floor tenant, who relocated about three years ago. “It was obvious. It was disguised as a massage parlor, but it was never open.”

The former tenant, who asked for anonymity, said he never saw any employees and the lights would only go on at weird hours of the night.

The business was one of the reasons he relocated.

The complaint obtained by The Post, filed in Manhattan Supreme Court, calls for a full one-year closure and $1,000-a-day fines against landlord Tsukiji KK Corporation and other named respondents.

A 911 call in October prompted an NYPD investigation into the house of ill repute, per the complaint.

On Dec. 3 and Jan. 7, an occupant at 57 Star Beauty Shop agreed to engage in manual stimulation of an undercover officer in exchange for a fee, the city claims.

57 Star Beauty Shop advertisements for “Sweet, sexy, hot Asian lady” and “Real Slim Busty Beauties Asian Models” were allegedly found at HarlotHub.com and SkipTheGames.com last week. The latter website was also used to bust NoMad Suites hotel in January.

Owner Melinda Her denied the allegations, insisting to The Post — via a friend who translated from an Asian dialect to English — that they only do facials, head therapy and massages.

The 57 Star Beauty Shop is on the second floor of 115 W. 57th St., a six-story, seven-unit walkup. Its legitimate massage services are splashed across the front window, and a sandwich board on the street promotes 30 minutes of “aroma relaxing bodywork” for $58. Another board offers a 30-minute head therapy special for $58. The first floor of the property is occupied by a deli, JIH Market.

To the east of the building is 111 W. 57th St. The ritzy building has a penthouse in contract for roughly $42 million, which would mark the priciest residential deal so far this year along the thoroughfare if it closes. To the west of the building is 117 W. 57th St., a small residential building with an Angelo’s Pizza at the base.

Some of the high-profile buyers on Billionaires’ Row, which is at the southern edge of Central Park, have been Ken Griffin, founder and CEO of Citadel; Michael Dell, founder and CEO of Dell Technologies; Bill Ackman of Pershing Square Capital Management; and Daniel Och, chairman and former CEO of Och-Ziff Capital Management.

A $128 million duplex at Central Park Tower, at 217 W. 57th St., the tallest mostly residential building in the world, hit the market in November.

Former tenants at 115 W. 57th St. were not surprised to learn of the suit.

A lot of men would traipse in and out of the building even a decade ago, according to Aimee Tan, a former resident on the fourth floor.

Tsukiji acquired the property in July 2009 for $5.8 million, per city records.

“We just had a feeling they were doing something similar” to the allegations in the current complaint, Tan said.

A representative for the building’s ownership entity could not be reached for comment.