Officers were called to Southwater Country Park, near Horsham, at 2.15pm after the body was discovered.
A spokesperson for Sussex Police said the body was ‘badly decomposed’ and was found ‘face down in woodland’.
The death is currently being treated as unexplained, though there appears to be no suspicious circumstances, police added.
Enquiries are being made to identify the man. The West Sussex coroner has been informed.
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