The victim reported being attacked in Priory Gardens near Court Road on Saturday evening (February 22).
Police say the girl was walking through the park when the alleged offence took place.
A spokesperson for Metropolitan Police confirmed today (February 24) a cordon remains in place and officers are still at the scene.
No arrests have been made.
The victim is being supported by specially trained officers.
A Metropolitan Police spokesperson said: “Police are currently investigating a serious sexual offence allegation reported to have occurred against a teenage girl on the evening of Saturday 22nd February, when she was walking through Priory Park in Orpington.
“A cordon remains at the location to preserve the scene.
“Enquiries are ongoing.
“The victim is continuing to be supported by specialist officers.”
