The teenager was assaulted in the bus station at Green Street, Kilmarnock, on Saturday night.
Police Scotland said the teenager and the group he was with were involved in a disturbance with another group, and that he was assaulted at the station at around 10.20pm.
The youngster was taken to Crosshouse Hospital for treatment to serious but non-life threatening injuries.
Detective Constable Paul Tracey said: “This was a serious assault which will leave the victim with life-long scars and we are keen to trace those responsible as soon as possible.
“I would urge anyone who may have been in the area, or at the bus station at the time and saw the disturbance, to contact police as soon as possible.”