Officers on patrol were flagged down by the victim in Regents Park Road just before 11pm on Friday (April 11).
The 16-year-old reported that she had been raped in the park.
She was taken to hospital by London Ambulance Service, and is being supported by specialist police officers.
A crime scene remains in place today (April 13). Pictures taken yesterday showed a large police cordon in place, with at least a dozen officers working at the scene.
No arrests have been made, but police enquiries are ongoing.
The incident came on the first day of the year that Royal Parks closed Primrose Hill to the public at 10pm.
These weekend night time closures, running until 6am in the morning, will be in place until October. The park remains open 24 hours a day during the week.
Royal Parks has been approached about the incident.
Anyone with information is asked to call police on 101 or message @MetCC quoting reference CAD 8380/11APR.
Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.
