The force confirmed he had sustained life threatening injuries during the brawl on Garratt Lane, but passed away in hospital.
Officers on patrol saw a large number of males fighting in the area just before 5pm.
A second youth, also believed to be 15, suffered stab wounds and a third was left with a head injury - neither of which has been deemed life threatening by paramedics.
Two men have since been arrested, one on suspicion of GBH, the other on suspicion of affray.
Inquiries into the incident are ongoing, a statement from the police force confirmed - with a section 60 stop and search order in place across Wandsworth in the wake of the incident.
Extensive road closures were set up on the busy thoroughfare alongside Southside Shopping Centre.
Anyone with information, video or images that could assist police have been urged to call 101 reference CAD 5042/29 Oct.