The address was cordoned off for hours as scenes of crime officers investigated the incident in Tarring Road, Worthing, from around 10.30pm.
A guard remains on scene today, according to police.
A spokesperson for Sussex Police said: “Police are dealing with an unexplained death in an address in Tarring Road, Worthing.
“A guard remains at the address. Scenes of crime officers have attended.
“A post mortem is scheduled for later today. Next of kin inquiries are being completed.”
The spokesperson said when the results of the post mortem are known more information will be available.