The tourist, Mervyn Michael, 58, was detained and is facing charges after he allegedly punched Vyacheslav Cherepanov, 48, causing him to lose a tooth.
Pattaya police and the volunteer rushed to Walking Street in South Pattaya after a food shop alerted police that a tourist had refused to pay his bill.
Police said the volunteer was punched when he tried to speak to the tourist about paying the bill. The tourist was taken to the police station while the volunteer was sent to Muang Pattaya hospital.