The theft happened at about midday on Tuesday, October 11, in Poundworld, when, after being distracted by a man and a woman, she realised cash was missing from her bag.
It is believed the thieves followed the woman from Barclays Bank, where she had withdrawn the cash, to the Grosvenor Centre.
Anyone with information is asked to contact Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.