A large amount of cash was stolen from the Cheque Centre in Foundary Walk after a man threatened a member of staff with an object pushed into her back.
The robbery occurred between 7.55 and 8.10am on Monday, November 10, when a member of staff arrived for work, only to be intercepted after she had unlocked the door by a man who pushed an object into her back, forced her to go into the shop and demanded she open the safe.
It is the third time the store has been targeted since April.
The man stole a large amount of cash, including sterling, Euros and US Dollars, before making off. The staff member was not injured in the robbery.
The offender is described by police as a man aged in his late teens to late 20s, of slim build, 5ft 10in to 6ft. He wore a long, dark hooded coat, dark trousers, white trainers and was carrying a dark coloured rucksack.
Detective Inspector Phil Mills, from CID, said: “It is very unusual to have three similar incidents take place at the same place in such a short period of time, and I am appealing for the public to help identify the offender.
“We would like to speak to anyone who may recognise the description of the offender in any of the incidents. We would also like to speak to anyone who may have been offered any Euros or US Dollars for sale.
“Extensive enquiries are taking place to identify the people responsible for these incidents which we believe are linked.”
The first robbery took place on Tuesday, April 22 and the second one on Monday, June 30.
Anyone with information can call Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555 111.
Officers are working with the shop to provide further crime prevention advice and support for staff.