The incident occurred at Tullibody Post Office, Stirling Street, on Monday 16 July last year around 8.30pm, when a man walked into the store and threatened staff.
The female shop assistant was not injured during this however was left badly shaken.
Detective Inspector David Mitchell said, "We are keen to trace the person in the image and we believe somebody may recognise the person in the CCTV images from the clothing being worn."
The person police are keen to trace is described as 16 to 40 years old, with a local accent, sallow skin, with dark eyes.
He was wearing a white hooded sweatshirt with black lettering across the chest and black tracksuit bottoms.
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.