The 33-year-old victim was “severely shaken” by the ordeal, according to officers who are now appealing for help to track down the culprit.
The robbery happened in the early hours of Thursday morning in the North Lanarkshire town of Bellshill, Police Scotland said.
At approximately 2am, the victim was attacked in the Coltness Drive area by the man with a knife.
His wallet was stolen – although later retrieved – and he suffered head injuries during the incident.
The suspect is believed to be in his early 20s and is of slim build with short, dark hair. At the time of the robbery he was wearing a light-coloured T-shirt.
Detective Constable Lindsey McIntyre of Wishaw CID said: “This appears to have been an opportunist attack and the man has been left severely shaken and with an injury to his head and face.
“This type of behaviour is completely unacceptable and it is vital that we trace the individual responsible.
“I would appeal to anyone who was in the area at the time and may have witnessed the attack, or seen anything out of place, to please get in touch.”
Anyone with information is asked to contact Wishaw CID by calling 101, quoting incident number 3557 of Thursday July 1 2021.
Alternatively, witnesses can call Crimestoppers on 0800555111, where details can be provided anonymously.