Between 9.30am and 10am on Saturday, May 18, the man had withdrawn the cash from an ATM near to Sheaf Street when he was approached by a woman claiming to be from a deaf charity, asking for donations.
The man donated £1 and signed his name on a sheet, with the woman then asking to check his signature, which prompted the man to take out his wallet.
The woman then placed the signature sheet over the man’s wallet and took the £100 from it, before leaving the area, walking through Foundry Walk, Bowen Square, and down to New Street.
Officers believe the woman pictured may have information about the incident, and are asking her, or anyone with information about her identity, to call Northamptonshire Police on 101.
Information can also be shared by calling Crimestoppers in confidence on 0800 555111.