Four men and three women are in custody. Scotland Yard said the arrests were made on “suspicion of various offences”.
Police were called to Prentis Road, South London, at 4.16pm on Thursday to reports of a stabbing.
Officers and emergency ambulance services attended the scene and found a male with a stab wound to the chest.
The victim was taken to a hospital in central London where he was pronounced dead at 4.51pm.
“Police believe they know the identity of the victim, who is in his late teens, and have informed next of kin,” a statement read.
Met Police have advised that officers remain at the scene and there are currently road closures along Prentis Road.
The public are advised to avoid the area.
The statement by the force said: “The investigation is in its very early stages and work to establish the circumstances continues.
“Anyone with any information is asked to call police on 101 quoting CAD 5897/10JUNE.”
“My thoughts and prayers are with his loved ones.”