The incident happened around 11am on Saturday on London Road near to Abercromby Street.
The 30-year-old man was threatened with a knife whilst sitting in his car, a blue Jaguar XF, which the suspect later stole.
The robber is described as white and of a muscular build wearing a black baseball cap and dark coloured jacket.
Police Constable Emma Laurie said: “The victim was left extremely shaken by this incident and we are working to establish the circumstances leading up to the robbery.
“If anyone has any information relating to this, or saw anyone matching the descriptions either before or after the incident, they should contact us.
“Anyone with information can call Police Scotland on 101, quoting incident 1481 of 26 October, or Crimestoppers anonymously on 0800 555 111.”