A Brooklyn man was knocked unconscious by a callous thug — who then took a selfie with the victim, according to police.

The 43-year-old victim was walking near the Shake Shack at Old Fulton Street and Water Street by Brooklyn Bridge Park at about 9:15 p.m. Thursday when an unidentified assailant clocked him with his fist, officials said.

The attacker was seen snapping a photo of himself and the victim before running off. He is described as about 16 years old and 6 feet tall, wearing a dark bomber jacket and short braids, according to cops.

The motive for the attack is unknown, police said.