The assault happened as Sheffield band the Arctic Monkeys were performing on Sunday night at the festival at Bramham Park, near Wetherby.
Police believe a group of three or four men may have witnessed what was happening. Det Supt Paul Taylor, who is leading the investigation, said: "I think in my own mind that they thought that they were watching a courting couple or a couple engaged in a bit of horseplay – but they were actually witnessing a very serious sex assault."
Mr Taylor said: "These men are not thought to have been involved in any way. They are witnesses and we are asking them to come forward.
"The attack occurred either whilst the Arctic Monkeys were on stage or soon after and it had just begun to rain. I'm asking people to think about where they were standing when the Arctic Monkeys were playing and who was around them."
Up to 70,000 people, from across the country, were at the annual event when the incident happened between 9pm and 10pm at a toilet block close to a Mexican food outlet.
The suspect was 20 to 30 and about 6ft with dark hair. He was wearing a dark coat.
Forensic examinations have been taking place at the scene and hundreds of hours of CCTV film will be studied.
Witnesses are urged to contact police on 0113 241 3259. People can also text any information to 07786 200 806, starting the text with the word Festival.