Police are appealing for information following a stabbing on a bus in Croydon on Friday afternoon 3 May.
The incident took place at around 4.10 p.m on the top floor of the No 60 bus.
The Air Ambulance service attended and the teenager was accompanied by a HEMS (Helicopter Emergency Medical Service) doctor to a south London hospital by a road ambulance vehicle also sent to the scene.
The bus had stopped in the London Road junction with Broad Green Avenue, Croydon and was heading towards Croydon Town Centre.
The 18-year-old victim is reported to be in a stable condition. No arrests have been made.
Police are appealing for information from commuters who were travelling on this bus between 4 and 4.20 p.m.