The incident happened between the Horsemarket stop and the Newland Street stop when the offender crawled under the seats to grab the 35-year-old victim by the ankle.
He left the bus at the Newland Centre stop.
The offender is described as white, in his 20s and 6ft 2in with dark, curly, messy hair.
He was wearing a blue fleece and black trousers.
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Anyone with information can contact Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.