A 19-year-old man was found dead in Prospect Park Lake Monday evening, according to police.

Cops say passers-by discovered him floating near the lakeside gazebo at the southeast corner of the Brooklyn park just before 7 p.m., and called 911.

Emergency responders rushed to the scene — but the teen was pronounced dead on arrival, authorities said.

The Medical Examiner’s Office had not yet determined a cause of death Monday night.

Officials said there were no immediate signs of trauma.

Law enforcement sources said that the victim had been reported missing before his death.