A 16-year-old boy has been arrested on suspicion of attempted rape and sexual assault.
The Spa at Gloucester Park was cordoned off after police received reports of the attack.
The attack is alleged to have happened at 11.00pm on Wednesday evening.
Police said a 16-year-old boy from Coleford had been arrested on suspicion of attempted rape and assault.
During the incident the 19-year-old woman was struck on the head during the attack and sexually assaulted.
She has received treatment for her head injuries and is currently being supported by specially trained sexual assault officers.
An immediate search of the area was carried out by response officers and the arrest was made shortly before 11.30pm.
Detective Inspector Ruth Mather, said: "This was a shocking and disturbing case that has left the victim badly hurt and completely traumatised.
"Along with the staff at the county's Sexual Assault Referral Centre we'll be giving her all the support we can.
"Forensic inquiries will continue at the scene and I would urge anyone with information who hasn't spoken to us yet to ring 101, quoting incident 488 of June 12.
"Crimestoppers can be contacted anonymously on 0800 555 111."
The arrested boy remains in police custody at this time.
The grounds have now been reopened by police.
