The teenager was taken to hospital by passers-by after he was knifed in Bensham Manor Road, Thornton Heath on Friday but died on Sunday.
Police believe he had clashed with two males on a moped who stabbed him multiple times.
It takes the number of teenage homicides in London this year to 22, including 19 stabbings.
The capital is on course to pass its worst ever total in a calendar year: 29 teenage homicides in 2008.
Acting Detective Sergeant Helen Ayling said the stabbing took place at about 3pm on Friday.
She added: “My thoughts and prayers are with the victim and his family.
“We believe the victim had been involved in an altercation in Bensham Manor Road with two males riding a moped. We are appealing for anyone who may have knowledge of this incident or who may have seen the moped or suspects concerned to come forward.
“We also need to hear from anyone who may have dash cam or security camera footage from in and around Bensham Manor Road at around 3pm on Friday afternoon.”
