Two men burst into the Ladbrokes in Jeffcock Road, High Green, at just after 4pm yesterday and demanded money from terrified staff.
Nobody was hurt in the raid and the pair made off with an undisclosed sum of money.
Eye witnesses reported seeing 'a Corsa parked up full of lads with their hoods up' outside and claimed one of them was armed with a gun.
A South Yorkshire Police spokesman said the incident log showed the men were armed with a screwdriver and a piece of metal.
He added the presence of a firearm cannot be ruled out at this stage as an investigation is ongoing.
The spokesman said: "We had a report from members of staff that two men entered with weapons and left the scene with a quantity of cash taken from the till.
"An investigation is ongoing to establish the circumstances.
"Police are reviewing CCTV footage and have spoken to neighbouring businesses.
"No arrests have been made."
A Ladbrokes spokeswoman said: "We can confirm an incident took place on Friday afternoon around 4pm and we are now assisting police with their enquiries."
Contact police with information on 101.