A stranger randomly slugged a 77-year-old man in the face in Manhattan, leaving the oldster with a fractured skull when his head hit the sidewalk, police said Tuesday.

The septuagenarian was walking by 300 Park Ave. when “an unknown individual punched him in the face” around 11 a.m. Monday, an NYPD spokesman said.

The man fell and hit the back of his head on the pavement.

There was no known motive and cops were looking for video, police said.

The man, whose identity wasn’t released, was taken to Mount Sinai Morningside with a skull fracture and bleeding on the brain, police sources said.