Police were called to a unit on Wilkins Street, Yagoona, after a concern for welfare report at 10.30am today.
The unit was alight when officers arrived.
Fire and Rescue NSW extinguished the fire and a man was found dead inside the apartment.
Three other people were treated for smoke inhalation.
Police are investigating the cause of the blaze.
Anyone with information or CCTV footage is urged to contact the authorities.
