Three/four masked robbers wearing balaclavas robbed the Post Office in Church Lane, Kislingbury.
The robbery happened at about 11.10am on Saturday when the men entered the post office and assaulted the shopkeeper. They stole the till and cigarettes and then forced the victim to open the safe.
They stole a quantity of cash from the safe before leaving the shop.
The offenders were described as wearing black clothing including balaclavas and gloves and all had a medium build.
The men left in a light coloured car, possible silver, with a partial registration of GY51 or GU51.
Officers believe this to be an isolated case and are investigating to identify those responsible.
Witnesses to this incident, or anyone with information, should call Northamptonshire Police on 101 - alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.