A 24-year-old man has been arrested after a 20-year-old woman reported being raped at the Latitude Festival at Henham, near Southwold in Suffolk.
Police were called at about 0700 BST on Sunday.
The incident is alleged to have happened in a tent on the staff campsite and the area has been cordoned off while investigations take place.
Police said they believed both people knew each other prior to attending the festival, which ends on Sunday.
The woman was being spoken to by specially-trained officers.
The man has been taken into custody at Lowestoft police station where he will be interviewed by detectives.