A robbery took place at the Esso service station on the A45 near Earls Barton early on Tuesday.
It happened between 12.10am and 12.20am at the service station on the eastbound carriageway.
Three men entered the shop through the front door, threatened a member of staff and forced him to lie on the floor.
They stole cash and cigarettes, which they put into to a large Jewson bulk-bag, before making their escape through the back door.
All three men were wearing dark clothing, balaclavas and gloves.
Officers are appealing for anyone who used the service station, or drove past on the A45, around the time of the robbery and who may have seen anything suspicious, to contact them.
Witnesses should call Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111.