The incident happened while was sitting in a bus shelter in the Drapery, opposite Debenhams, at 4.30am on Tuesday, January 1.
A man sat next to the woman and started a conversation before putting his hand down her jeans.
The man is described as Asian, slim, and aged about 50, 5ft 3ins to 5ft 5ins, He was wearing a suit-type jacket, dark-coloured trousers and dark, pointed shoes.
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101. Alternatively, they can call Crimestoppers anonymously on 0800 111222.