The incident took place on Merton High Street during the early hours of Saturday (August 23).
Officers were called at around 3.30am.
The victim is being supported by specially trained officers and a crime scene remains in place on the High Street.
Forensics have been seen working in the area over the last few days.
No arrests have been made at this stage.
Police are urging anyone with information or who was in the area at the time to call 101 and quote 1269/23AUG.
