He struck at the BP petrol station at Meadowhead at 1.30am yesterday.
South Yorkshire Police said he threatened the manager with a gun before stealing £100 from the till.
He then ran from the petrol station across a nearby cricket pitch towards Lowedges.
A customer chased the suspect in his car but lost sight of him.
The robber was white, tall and wore black clothing, trainers and a balaclava.
Witnesses or anyone with information should call South Yorkshire Police on 101 and quote incident 50 of May 30.