The attacker demanded cash from the victim at the Morrisons ATM in Victoria Promenade at about 3pm on Tuesday, October 29.
Another man came to help her and the suspect ran off towards Derngate, police said.
The suspect is described as an Asian man, aged 25-35 with a black stubble beard.
He was wearing black jogging bottoms, a blue coat and a grey hoody.
Anyone who witnessed the incident, or has information about it, should call Northamptonshire Police on 101, or Crimestoppers anonymously on 0800 555111.