A gang of men wearing balaclavas attacked a village shopkeeper before escaping with the till, cigarettes and money from a safe.
The robbery happened at the post office in Church Lane, Kislingbury, at about 11.10am on Saturday, 14 September.
A spokesman for Northamptonshire Police said: “Three or four 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 men wore black clothing, balaclavas and gloves and all had a medium build.
They left in a light coloured car which was possibly silver and had a partial registration number of GY51 or GU51.
Anyone with information can call police on 101.
Alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.