A retired NYPD officer was socked in the back of the head in Brooklyn Tuesday in what is being eyed as a possible “knockout game” attack, police and sources said.

The 66-year-old former cop was walking on a sidewalk along Brighton Beach Avenue when a group of suspects came up and clobbered him in the back of the head just before 1 p.m., police and the sources said.

The suspects — who look to be teenagers or young adults — then quickly ran off towards Brighton 7th Street.

The punch was completely unprovoked, according to the NYPD.

A police source said the assault may have been part of a twisted challenge that dares players to slug a stranger on the street with the goal of knocking the person unconscious with a single blow.

The so-called “knockout game” terrorized New Yorkers a few years ago with groups of teens and young adults slugging unsuspecting victims and filming the attacks which they later uploaded to social media.

In Tuesday’s attack, the suspects walked up to the retired police officer while laughing before cracking him in the back of the head, the source said.

The victim — who sources said had been assigned to the 78th Precinct before retirement — suffered pain and swelling, but refused medical attention.