A 59-year-old man plunged to his death from the rooftop sundeck of a 36-story high-rise on the Upper East Side Saturday night.

He landed on the roof of the adjacent, four-story, East Side Middle School on East 91 Street and First Avenue.

Police did not immediately know whether he jumped, fell or was pushed, but a source said it looks like a probable suicide.

The name of the victim was withheld pending family notification. It was not known whether he lived in the luxury building.