The incident happened between 8.40pm and 9.20pm on Sunday in the park between Union Road and Summers Street.
The victim, who is 19, is being supported by specialist officers.
No one has been arrested at this time.
Officers investigating the incident would like to hear from anyone who was in this area of Northam on Sunday night, particularly the area between the Prince of Wales pub, Union Road and Summers Street.
Detective Sergeant Sean Owens said: ‘We are in the early stages of the investigation and in order for us to find out the exact circumstances what happened we would like to hear from anyone who was in this is area at the time the incident occurred.
‘It was still light when the attack happened and I believe someone to have seen a man or woman in this area of Northam.
‘No matter how small your information, it could be significant to our investigation, so please do get in touch.
‘We have increased patrols in the area, so if you see us out and about or have any concerns or information about the incident that could help, please do approach us.’
Call 101 and quote 44190284325.
Alternatively, call Crimestoppers on 0800 555 111 and leave information anonymously.