The incident happened on board a number 1 bus travelling eastbound in Church Road, Hove, at 9.15am on Wednesday, October 9.
A passenger on board was reportedly assaulted and passengers left the bus, which was at a stop between the Sackville Road and Connaught Road junctions.
Police officers want to speak with passengers on board, in particular one passenger who was reportedly assaulted and left the area.
The attacker is described as a man in his 40s, 5ft 5in tall, was wearing a green T-shirt, grey tracksuit bottoms and a black baseball cap.
Anyone with information should report it to Sussex Police online or on 101, quoting serial 380 of 09/10.
