Police were called at about 1.40am on Monday, 25 April to reports of a disturbance at a residential address on Delaford Road, SE16.
Officers attended and forced entry. Inside, they found four people suffering what are believed to be stab injuries.
Despite the efforts of emergency services, all four people – three women and a man – were pronounced dead at the scene.
Officers are in the process of contacting next of kin. Post-mortem examinations will be arranged in due course.
A man has been arrested on suspicion of murder and taken to a south London police station where he remains in custody.
Detectives from Specialist Crime are investigating.
It is thought that all five people were known to each other. Enquiries continue to establish the circumstances.
