She was approached by a man who touched her inappropriately as they passed each other in Hervey Street at about 2.15pm on Friday, July 5.
The victim shouted at him to get off her and he made off towards Carey Street and Clare Street.
The offender is described as an Asian man in his late 50s, of slim build, about 5ft 1in, with thinning, short dark hair.
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He was wearing a black, long-sleeved jumper over a white shirt with black trousers and black shoes.
Witnesses or anyone with information is asked to contact Northamptonshire Police on 101.
Information can also be reported anonymously to Crimestoppers on 0800 555111.