A gun-wielding robber stole cash from a pub in a village near after he locked staff behind the bar.
The man entered the Boat House Pub in Braunston just after midnight on Saturday, and demanded money.
No one was injured in the robbery and the man left the pub as soon as he had stolen the cash from the till.
Police would like to speak to anyone who may have seen a man acting suspiciously at around 11.45pm on Friday, on the canal towpath near the pub or at the bus stop on the main road next to the pub before the robbery.
The man was of heavy build, out of breath and wearing dark coloured clothing.
Detective Sergeant Gavin Suttie from Northampton CID said: “This is a rare incident to take place in Braunston and we are appealing for the public’s help as part of our investigation.
“We would like to speak to anyone who may have witnessed this incident or any suspicious activity near the pub or in Braunston late on Friday night and early on Saturday morning.
“Witnesses or anyone with information about this incident should call police on 101 or Crimestoppers anonymously on 0800 555 111.”