The woman, aged in her 20s, was attacked after leaving the event's Chinawhite enclosure early on Sunday, with men she met there.
She walked with them to the Harpsden Road and Reading Road area and was raped outside a car, which the men then drove off in.
A passerby found the victim near the Three Horseshoes pub shortly after.
Daniel Grist, secretary of the regatta, said: "We have been liaising with Thames Valley Police, regarding this terrible incident, which took place after Regatta facilities had closed and racing had finished for the day on Saturday 1 July.
"The Chinawhite area is not located on land managed by Henley Royal Regatta but we will continue to assist Thames Valley Police and are working with them to secure CCTV footage to assist in identifying the perpetrators."
Thames Valley Police is appealing for witnesses, and specially-trained officers are supporting the woman, who left the Chinawhite enclosure between 00:00 and 02:30 BST.
Det Ch Insp Lis Knight said such incidents were rare in the area.
The Â£3m rowing regatta sees more than 200 races take place on the river Thames over five days.
The annual regatta, which has been held since 1839, was expected to attract 200,000 spectators to this year's event.
Organisers of the Henley Royal Regatta are supplying hours of CCTV footage to police after a woman was raped.