The 18-year-old man died in the early hours after being found injured in Denmark Road, Lambeth, south London, shortly before 7pm on Wednesday.
In a statement, the Met Police said the victim was on a moped with someone else who left him and drove away.
Members of the public tried to save him, giving first aid before emergency services arrived. However, he died in hospital a few hours later.
It is the latest in a string of violent deaths in the capital since the start of the year.
Officers have launched a murder inquiry.
Detective Chief Inspector Simon Harding said: “This is a tragic incident which has resulted in yet another young person being fatally stabbed in London.
“We are appealing for anyone with information to come forward. We need to trace the moped and also the person who brought the victim to the scene – it’s important we establish where the victim initially received his injuries.
“Anyone who saw the moped make off from the scene, we equally need to hear from them.”
So far, there have been no arrests.
The teenager’s next of kin have been informed, police said, but formal identification has not yet taken place. A post-mortem will take place “in due course”.
:: Any witnesses or anyone with information should contact the Met’s homicide and major crime command on 0208 721 4205, or call 101, quoting CAD 6605/25 July.
Information can also be reported anonymously to Crimestoppers on 0800 555 111, or online at crimestoppers-uk.org