Police said the vulnerable man was targeted by two "despicable individuals" in East Kilbride on Friday night.
They threatened their 32-year-old victim with a knife before punching him and robbing him.
Det Con Stuart Burnside appealed for help in tracing the men.
He said: "These despicable individuals preyed on a vulnerable man who clearly could not stand up for himself and has been left absolutely traumatised by what happened.
"This type of abhorrent behaviour will not be tolerated and extensive enquiries are underway to find these criminals and hold them to account for their cowardly actions.
"I would appeal to anyone who was in the surrounding area late on Friday night, who may have witnessed the incident or may have seen two men matching the descriptions, to please get in touch."
The men approached their victim at a bus stop near Calderwood Square at about 23:00 on Friday. They were last seen heading towards Maxwellton Road.
Both men were white and in their mid teens or early 20s. The first was about 5ft 6in, and he was wearing a white T-shirt and dark coloured bottoms.
His accomplice was slim and about 5ft 8in. He wore a dark coloured tracksuit.
Their victim was treated at Hairmyres Hospital following the incident.
A man with learning difficulties has been left "absolutely traumatised" after being punched and robbed at a bus stop.