A man threatened staff at William Hill in the town's Stirling Street just after 21:00 on Friday before making off with a quantity of cash.
He is described as white, aged between 20-30, and about 5ft 9in tall with a stocky build.
He was wearing a khaki jacket with orange or red vertical lines under the arms and a green scarf over his face.
He was also wearing dark blue jeans, gloves and had a carrier bag.
He is believed to have been in the Herbertshire Street area a short time later.
Det Insp Michelle Findlay said: "No-one was injured but this was a frightening experience which left a staff member understandably very shaken.
"As part of our ongoing investigation, we'd urge anyone who was in the area around this time and saw the man to get in touch.
"Anyone who recognises his description or has any other information which may be able to help is also asked to contact us."
A robbery at a bookies in Denny was a "frightening experience" that left staff "very shaken", police have said.