A bus driver was dragged to the floor and kicked by youths after he told them to get off for being rowdy.
The incident happened on a McGill's service in Brediland Road, Paisley, at about 22:45 last Friday.
The driver was taken to the Royal Alexandra Hospital where he was checked over by medics and treated for bruising.
Police are checking CCTV and appealing for anyone who was on or near the bus to come forward.
The three suspects were heading towards Paisley town centre when the incident happened, close to Cross Road.
'Concerns about behaviour'
A Police Scotland spokeswoman said: "The bus driver asked a group of males to leave the bus over concerns about their behaviour. As they left one of the males punched the driver to the face.
"He was then punched and kicked by the other males as he lay on the ground.
"The victim did not sustain serious injuries during the attack but was treated at the Royal Alexandra Hospital in Paisley for bruising.
"We are appealing to anyone who may have been on or near the bus who may have witnessed the attack to get in touch."