The 53-year-old was threatened while walking in a park in Blantyre, South Lanarkshire.
The thief fled with his phone and a small amount of cash after brandishing a knife. The incident happened at around 8.50pm on Tuesday night.
Police said the man was not injured but was badly-shaken after his ordeal.
The suspect is described as white, in his early twenties, 5ft 10 inche and of slim build.
He was wearing a black jacket with a grey stripe across the chest, a grey scarf around his face, and slim fitting blue jeans.
Detective Constable Edward Mather of Hamilton Police Office said: "I would urge anyone who witnessed this incident take place, or who may recognise the description of the young man responsible, to contact Police Scotland on 101 or Crimestoppers on 0800 555 111, anonymously."