Police were called after the incident at McColl’s in Farmstead Road at 10.40pm on Monday (May 6).
A man entered the store and demanded money and cigarettes from the cashiers. He put the money and cigarettes into a white carrier bag before fleeing.
A Northamptonshire Police spokesman said: “The man is described as white.
“He wore a black balaclava and spoke with a Corby accent.”
Reports on social media said the robber held the cashiers up at gunpoint.
A police spokesman said the attacker had a weapon but that they were not in a position to confirm the nature of it yet.
The spokesman added: “Officers would like to hear from anyone who was in the area of the Farmstead Road shops at around the time of the incident who may have seen the man before or after he entered the store.
“Anyone with information about the robbery should call Northamptonshire Police on 101, or Crimestoppers in confidence on 0800 555111.”
No arrests have been made and a crash outside the Thursfield chemist in Corporation Street is not being linked.
The shop declined to comment.