Officers were called to the area of the pedestrian subway between Riverside and Coppins Bridge, just off Snook’s Hill, sometime before 23:30.
The subway tunnel and surrounding open land was sealed off at both ends for a time with Police maintaining a scene guard.
All cordons in the area have now been lifted but investigations continue.
A spokesperson for Hampshire Constabulary has told Island Echo:
“We are investigating an incident of serious sexual assault against a 15-year-old girl which was reported on Saturday 22 February.
“The incident happened in an area near Snook’s Hill in Newport.
“At this very early stage of the investigation, we are trying to ascertain the exact circumstances of what happened and supporting the individual concerned”.
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Members of the public shouldn’t be alarmed as Police say that they do not believe there is a risk to the wider public.
Enquiries are ongoing.
Anyone with information relating to this incident should call 101, quoting 44200068540.