The attack happened after a 21-year-old man stopped his Vauxhall Corsa to speak to another driver, believed to be a woman, who was driving aggressively and erratically behind him in Main Street, Plean on Friday at around 11.45pm.
When the victim pulled over and approached the car, thought to be an old style dark-coloured Corsa, the passenger got out and assaulted him.
Police say inquiries are ongoing to trace the male responsible and anyone with information is asked to come forward.
Detective Kenny McAndrew from Stirling CID said: “The victim sustained a cut to his head following this attack, which resulted in him requiring treatment at Forth Valley Royal Hospital.
“Our inquiries are continuing to trace the suspect and the driver of the vehicle, who is believed to be a blonde-haired woman in her twenties.
“Anyone who can assist in identifying these individuals, or who has information relevant to this investigation is urged to contact police immediately.”
The suspect is described as white, around 6ft 2ins with a stocky build and shaved head. He was wearing a dark grey Adidas jogging suit with black stripes down each arm.
Anyone with information is asked to contact Stirling CID on 101 and quote incident number 4440 of August 11. Alternatively, an anonymous report can be made to the charity Crimestoppers on 0800 555 111.