The teenager reported seeing the act of public indecency whilst on the number 202 Dewsbury to Leeds service on Sunday, February 21.
The incident took place at around 11.30am while the bus was at Bywell Road in Dewsbury.
Police have arrested a 72-year-old man from Leeds in connection with the “lewd” act and six other public indecency incidents in Leeds city centre on February 23.
He has been released on bail pending further enquiries.
And officers are appealing for more information about what happened.
Sergeant Pat Kenning, of Leeds City Neighbourhood Policing Team, said: “There were a number of other people on the bus at the time and we would like trace them as potential witnesses to this incident.”
Anyone with information is asked to contact Sgt Kenning at Leeds Central Police Station on 0113 241 3833 or via 101.