Thieves flagged down a bus as it was travelling through a village and demanded the driver handed over cash.
The incident happened around 9 p.m. on Wednesday in Maddiston.
The bus, which had no passengers on it at the time, was heading along Vellore Road towards Main Road when it was stopped.
Detective Inspector Douglas Anderson said: “The man who flagged down the bus then got on board and threatened the driver with a weapon. He stole the takings, which is believed to be a two-figure sum, and ran off in the direction of Whitecross.
“A second person was seen at the time and was wearing a black or dark brown puff-style jacket.”
He added: “The driver was not injured but was obviously shaken by this. We appeal for anyone with information to come forward.”
The man police are keen to trace was described as 5’5”- 5’8” tall and was wearing a brown three-quarter length duffle coat with black combat trousers with white marks on the knees which appeared to be paint.
Anyone who with information can contact Police Scotland on the non-emergency number 101 or information can be given anonymously to Crimestoppers on 0800 555 111.