A cashier was threatened with a knife when two men ran into Rushden Service Station and stole a quantity of cash from behind the counter.
She was uninjured but extremely shaken by the incident, which happened at 10.20pm on Sunday (July 31) at the premises in Wellingborough Road.
Both offenders ran off into Wellingborough Road towards Windmill Road.
They are described as white and of medium build, about 5ft 9in and both wore grey hoodies.
Anyone who witnessed the robbery or has any information about it can call Northamptonshire Police on 101 and ask for Wellingborough CID, or contact Crimestoppers anonymously on 0800 111555.