The incident is believed to have happened at around 7.30pm on Saturday, July 21, on the bridge close to the railway station.
A spokesman for Northamptonshire Police said: "Police are seeking the owner of a bike who may have been threatened in an alleged attempt to steal his bike.
"The cyclist was on a red and white mountain bike and rode off following the incident. Officers are asking him to make contact. "
Anyone with any information can contact Northamptonshire Police on 101.
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Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.