The raid happened at the Murco garage in Bowhouse, Maddiston, at about 21:45 on Friday.
The man, who was wearing a balaclava, threatened staff with a weapon and demanded money.
He made off with a number of items. Officers have appealed for witnesses to come forward.
The suspect was described as 5ft 8in to 5ft 10in tall and of stocky build. He spoke with a Falkirk accent.
He was wearing a grey camouflage jacket, dark trousers, dark trainers with white soles and black gloves.
Det Sgt John Burgoyne said: "Although no-one was harmed during this robbery, it was still upsetting for those who witnessed it.
"It is essential that we trace this man as soon as possible. Anyone who has information, or recognises the description of the man, is asked to contact Police Scotland immediately."
Police are hunting a masked armed robber who held up staff at a petrol station near Falkirk.