A skimming device was attached to an ATM outside Barclays Bank in Rushden.
Two men fitted the device to the cash machine, in College Street, at about 6am on Friday, January 30, and one of them returned to remove it at about an hour later.
During that time several people used the cash machine, including one person who is believed to have seen the offenders.
Both men were Asian, one wore a light coloured coat and dark trousers and had dark hair and a slight beard.
The second man also wore a light coloured coat with lighter coloured trousers and also had dark coloured hair.
Witnesses should call the police on 101 or Crimestoppers anonymously on 0800 111 555.