Two men and a woman - all aged 20 - were walking in the Ferguslie area of Paisley at around 1am on Thursday.
The group were then confronted by two males who threatened them and demanded cash.
However, after assaulting one of the 20-year-old men, the suspects made off with a watch.
Detective Constable Craig Riddell, said: "This was a completely unprovoked attack and I am keen to trace those responsible.
"When the men were being assaulted, the woman stopped a passing motorist and asked for help. The suspects ran off and the motorist gave chase before losing them.
"It is believed that two males, possibly aged in their 20s were in the car but at this time, the make and model is unknown.
"I would ask the occupants of the car to come forward."