A man threatened staff at McDonalds, Sixfields, late on Saturday night as he stole cash.
The man entered the restaurant at about 11pm on Saturday, 6 August and threatened staff with a hammer and demanded cash.
A small amount of money was handed over to the man who made off without injuring anyone.
The offender was a white man, tanned, aged between 20 and 25, of slim build and about 5ft 11in. He was English, possibly drunk and had a large graze down the back of his left leg.
He was wearing a black hooded jacket with the hood up, black sunglasses, light grey shorts, white socks and black trainers.
Detectives 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 contact police on 101 or Crimestoppers anonymously on 0800 111 555.