Nassau County police say they got a 911 call around 4 p.m. Sunday for a body slumped over a log in the Mill Pond Park Preserve in Wantagh.
Investigators and K-9 units searched a wooded area through the night for evidence.
It is not clear how the man died.
The body was transported to the medical examiner for autopsy.
Police are asking anyone with information about the death to contact Crime Stoppers at 1-800-244-TIPS.