The robbery took place at the Spar store in the South Lanarkshire town's Alberta Place at 1.55pm on Sunday, August 18 last year.
The man is described as white, aged between 30 and 40, thin, pale and with a clean shaven face and head.
He was wearing a khaki jacket, black trousers and white trainers.
He threatened a female member of staff before making off with a three-figure sum of money and cigarettes.
Anyone with information is asked to call the non-emergency number 101.