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.

57 Star Beauty Shop, 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.”

57 Star Beauty Spa stands on one of Manhattan’s most expensive blocks — and is the subject of a recently filed lawsuit. Brian Zak/NY Post

57 Star Beauty Spa listed its alleged sexual services on HarlotHub.com.

The main website for 57 Star Beauty Spa — offering treatments such as facials — remains active.…