The victim, a 17-year-old girl, reported that she had been touched on her body by a man who walked past her outside the Brunswick Centre yesterday afternoon.
Now North Yorkshire Police are seeking witnesses to the assault, which took place in York Place at around 4.20pm.
A spokesman said: "We are conducting enquiries and are requesting the public’s help to establish full circumstances surrounding the incident.
"In particular, we are appealing for information from anyone who observed the incident or recalls seeing the man."
The offender is described as black, aged in his 20s, about 5ft 9in, of a large or broad build, with short, black hair.
Anyone with information is asked to email [email protected] or call 101, select option 2, and ask for David Hinchliffe.
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
Please quote the North Yorkshire Police reference number 12170149734.