CCTV has been released of two men police want to trace in connection with the robbery at Betfred in Granton Walk, Ingol.
Police said that at around 9.15pm on Sunday, April 17, two men approached the bookmakers.
One of the pair held the door while the other entered the shop and threatened staff with a large knife while making demands for cash.
DC Mark Burns from Preston CID said: “This was clearly a very frightening ordeal for the people working in the shop at the time.
“Thankfully no one was physically hurt during the incident despite a knife being used make threats.
“If you saw or recognise these men, or you have any information about the incident, please get in contact with us.”
The offender managed to get away with around £4,000 in cash.
One man is desctibed as white, in his early 20s, round 5ft 5 ins and was wearing a light grey hooded jacket and light grey tracksuit bottoms.
The other man is also white, of a similar age, of slim build and was wearing a dark blue jacket, blue tracksuit bottoms and a black balaclava.
Anyone with information can call Preston CID on 01772 209755. Alternatively they can ring police on 101 quoting log 1408 of April 17.