The robbery happened at around 6.40pm on Friday at the William Hill shop on Maryhill Road in Glasgow.
The suspect threatened staff with what appeared to be a firearm and demanded money.
Police said no firearm was discharged and no one was injured, however the two staff members - a 20-year-old woman and a 21-year-old man - were "badly shaken".
The man is described as white, 5ft 11in and of slim build. He was wearing a black hooded top with a white zip, black jogging trousers and white trainers. He had a black scarf wrapped around his face and was carrying a drawstring bag.
Detective Constable David Somerville, of the Greater Glasgow Division's robbery unit, said: "Maryhill Road is a particularly busy area and I am sure that there are motorists or pedestrians that may have seen this man acting suspiciously near the bookmakers..
Meanwhile, in a separate incident, police in Edinburgh are searching for a man who robbed a post office in the city while wearing bright purple gloves.
The incident happened at around 4.45pm on Friday at the Dalry Road branch.
The man assaulted two members of staff before managing to get away with a three-figure sum of money.
He ran into Dalry Road and on to Orwell Place, and was last seen turning into Caledonian Crescent.
He is described as white, between 30 and 40 years old, and 6ft tall with a slim build.
He was wearing a dark woollen Tammy hat, dark scarf, dark bomber-style jacket, blue jeans and dark trainers.
Detective Sergeant Neil Spowart said: "This was a terrifying ordeal for the staff, and although they were not physically hurt, it is imperative we catch this man as soon as possible.
"The area was busy at the time and I am appealing for any member of the public who saw the man, either in West Park Place before the robbery, or as he made his escape."