Police cordoned off the Henry Bell Street scene and it is understood officers remained until early afternoon on Sunday, March 22.
Eyewitnesses reported seeing forensics teams in operation.
A statement from Police Scotland said there was ‘no threat’ to members of the public.
A spokeswoman for Police Scotland said: “Around 3.40am on Sunday, March 22 2015, police received a report of an 18 year-old woman assaulted in Henry Bell Street, Helensburgh a short time earlier. “Enquiries have been ongoing in relation to this report however there is no threat to any members of the public.”
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