Essex Police were called to reports of a man unwell inside a property in Kursaal Way, Southend, at about 12.10am on Thursday.
A man in his 50s was declared dead at the scene and police appealed for information.
A 48-year-old man has been arrested on suspicion of manslaughter and was taken into police custody.
Det Ch Insp Louise Metcalfe said: “I know this will be a concerning incident for people living in the area but we believe those involved are known to each other, it happened inside a private address, and there’s no risk to the wider public.”
A heightened police presence would be visible in the area, she added.