Police were called an apartment complex, on Castlereagh St, in Haymarket, at 3.15am on Sunday morning after reports a man was lying unconscious in the hallway of the 13th floor.
NSW Police say when they arrived, the man was unresponsive.
Members of the public had tried CPR but he was pronounced dead at the scene.
Police say they were told the man was found injured after a fight was heard in the hallway.
The 20-year-old man was arrested nearby and taken to Day Street Police Station where he was later charged with murder.
The alleged offender is currently receiving treatment at St Vincent’s Hospital, under the guard of police.
He is in hospital having treatment.
A crime scene has been established as inquiries continue.