At about 8.30pm on Wednesday 24 August, a black man and the white woman parked a white van at Goring Gap and then walked a short distance, across the greensward, to the beach where the woman was raped.
The pair then returned to the van and drove off.
A 42-year-old man was later arrested on suspicion of rape. He was interviewed and released on bail until November 18 while enquiries continue.
The woman did not require hospital treatment but is receiving welfare support from specially trained staff.
Detective Constable Owen Watkins said: “If you saw what happened or have any other information about the incident, please contact us via [email protected] or call 101, quoting serial 1741 of 24/08.”
Members of the public can also contact the independent charity Crimestoppers anonymously on 0800 555 111 or visit www.crimestoppers-uk.org.