The victim has been taken by air ambulance to a west London hospital.
In a statement the Met Police said: “At 6.04pm on Wednesday, 20 March, police were called to reports of an injured male at Balham Bill, SW12.
“Officers, London Ambulance Service and London’s Air Ambulance attended.
“At the scene an 18-year-old male had suffered a stab wound.
“Officers from South West Command Unit investigate.”
Details of the victims condition are currently unknown.
No arrests have been made so far.
A crime scene remains in place whilst police investigate the incident.
Last week Chancellor Philip Hammond announced an additional £100 million in funding in a bid to tackle the knife crime crisis.
The Government has been under pressure to increase police funding following a spate of fatal stabbings.
However London mayor Sadiq Khan described the money as “a drop in the ocean”.
He commented: “Cuts have consequences and the government needs to urgently give our police the funding they desperately need.”
Total funding for forces in England and Wales reduced by 19% in real terms from 2010/11 to 2018/19, according to the National Audit Office.
Officer numbers have fallen by nearly 20,000 since 2010.
Anyone who witnessed this incident or has information is asked to call South West Command Unit on 101 quoting CAD6174/20MAR, Tweet @MetCC or call Crimestoppers anonymously on 0800 555 111.