A 16-year-old boy was slashed on the staircase of the luxury Upper West Side high-rise Wednesday night by a crew of four attackers, according to law enforcement sources and a neighbor.

The knife attack unfolded at about 10:45 p.m. inside Waterline Square, where residents are offered a number of flashy amenities and market rent for a one-bedroom unit starts at around $7,000 per month.

“It’s crazy though, there are millionaires that live here, then there are these young kids coming here from the projects, hanging out, pretending to be gangsters or whatever,” building resident and witness to the attack John Him, 23, told The Post.

“They leave me alone pretty much, but I always check on my Caucasian friends to make sure they’re OK,” said Him, who is black.

The youngster was slashed numerous times in the back when he was cornered by the four men in a stairwell at the building on West 61st Street, according to law enforcement sources.

He was also knifed in the left wrist and left shoulder and was taken to Mount Sinai Hospital in stable condition.

The perps fled on foot and are still on the lam, police said. It’s unclear how old the attackers are.

Him said the victim lives in the building and he’s seen the boy around other times.

“I saw some kids play fighting, a group of them, well I thought they were play-fighting, but they were fighting,” Him said. 

The victim ran inside as he was being chased by the suspects and hid behind the front desk in the lobby for a short time, Him recalled.

“The one kid was waving the knife around, handheld s–t, not like a sword, it happened really fast, like five seconds, they slashed him across the right side, under his arm. I saw the blood on the floor,” he said.

Waterline Square offers wide-ranging fitness facilities, including squash and a rock climbing wall, creative studios, a children’s playroom, and a screen room, according to the building’s website.

It does include several affordable housing units as well.

Other building tenants were left stunned by the slashing.

“Maybe we need to think about more security in the building, but we’ve been here for a long time, and everything’s always been great,” said a man in the building who did not want to give his name. “So, this was obviously a surprise.”

The man’s wife said she has seen “groups of riff-raff kids” hanging out around the building because there is a nearby park that is publicly accessible to anyone.

“I was just saying to [my husband] that I was walking home from downtown and I felt like totally safe on the subway and everything, then I came home and I felt unsafe in my building when I was walking in. And I was like, that doesn’t make sense,” she said.

The luxury building attack was among a rash of teen violence to again plague the Big Apple in the past day.

A 14-year-old boy was shot and wounded in Cypress Hill, Brooklyn, around noon Wednesday, authorities said.

He was struck in the left forearm and was taken to LIJ Medical Center in stable condition, according to police.

Two male suspects fled and no arrests have been made.

A 19-year-old man was shot in the left shoulder about 12 hours later on the grounds of a Brooklyn NYCHA complex in East New York, cops said.

He told investigators he was just standing there around 12:35 a.m. when gunfire erupted, the NYPD said. It’s unclear if the victim was the intended target, and he was taken to Brookdale Hospital in stable condition.

No arrests have been reported by police so far.