A man was discovered dead on the third-floor staircase of an Upper West Side brownstone, police said Sunday.

A resident of the building returned home at 12:20 p.m. Saturday to find the 46-year-old man’s body lying unresponsive on the third-floor landing of 47 W. 90th St., police said.

Preliminarily, the incident appears to have been accidental, though the cause of death will be determined by the city medical examiner, authorities said.

The man did not live in the nine-unit, four-story building, which is located between Columbus Avenue and Central Park West, police said.

The identity of the victim has not been released pending family notification.