Police in Philadelphia are looking to identify a teen who sucker punched a mentally-challenged man as a group of the punk’s friends laughed during the senseless attack.

A 30-second, slow-motion video of the attack posted on Monday to Facebook shows a young unidentified teen sizing up the victim before landing a vicious right-hand punch in front of a group of boys — one of whom laughing hysterically as the man recoils in shock and pain.

“This guy is a friend of mine who happens to be mentally challenged,” Gerald Andrews Sr. wrote on Facebook.

Andrews, 48, told Philly.com that the victim in the “heartbreaking” attack, which occurred at Chelten Avenue and Knox Street in the city’s Germantown section, works with him at a supermarket in Cheltenham and identified him as Mark.

“He keeps that smile on his face,” Andrews said. “He wants to talk sports. He’s a good guy.”

The video had been viewed more than 1.5 million times by Tuesday morning and was shared more than 41,000 times.

Andrews said a co-worker alerted him to a video of the attack online — but doesn’t know where it was originally posted. He made a copy and put it on his Facebook account in an effort to help police identify the brute.

“Obviously, what is depicted in the video is very disturbing,” police spokesman Sgt. Eric Gripp told Philly.com. “We would implore anyone who has info on the video, or who knows the identity of the victim to reach out to police.”