A man demanded cash from a petrol station worker in Daventry as he threatened them with a knife, before fleeing from the shop.
The robbery happened in Braunston Road petrol station, at about 9.10pm on Sunday, February 4, police revealed this morning.
A Northamptonshire Police spokeswoman said: "The offender was a black man, slim, about 5ft 11in and in his early 20s. He wore a dark-coloured, padded jacket with a hood, dark-coloured jeans, and white trainers with dark stripes on the side.
"He was also wearing a blue and white football scarf around his face throughout the incident."
Officers would like to speak to anyone who may have witnessed this incident or who may have any information regarding it.
Witnesses or anyone with information about this incident can call police on 101 or Crimestoppers anonymously on 0800 555 111.