The incident took place on Morden Hall Park on Monday (April 14).
The victim-survivor is currently being supported by specialist officers.
A 16-year-old boy was arrested on suspicion of serious sexual assault and currently remains in police custody.
A police investigation is currently underway.
Police are appealing for anyone with information to come forward, and are urging anyone who was in the vicinity of the park near the exit on Morden Road opposite the medical centre at around 6pm on April 14 to contact them.
Information can be reported by contacting 101, quoting the reference 01/7379934/25.
Information can also be reported anonymously by contacting Crimestoppers on 0800 555 111.
