Police are searching for a man believed to have sexually assaulted a 21-year-old passenger while on a bus between Ballarat and Melbourne earlier this month.

Victoria Police spokesman Leonie Johnson said the victim was on a bus that was replacing train services about 4.30pm on Saturday, December 3.

An image of a man who may be able to assist with police inquiries. Credit:Victoria Police

He told police when he got onto the bus, the offender moved seats so he was seated next to him before making inappropriate comments.

The offender then began touching him, he said.