An 18-year-old female reported the offence to Merseyside Police after being on a number 10A Arriva bus at around 7am on Saturday, October 1.
CCTV and witness enquiries are ongoing to identify the man responsible, and detectives have issued an image of a man they believe could assist with their investigation.
Detective Inspector Chris Saidi said: “Everyone should be able to use public transport without the fear of being subjected to an incident like this.
“This young woman was left shocked and upset by this incident, and we are determined to bring the man responsible to justice. We think the man pictured may have vital information that could help our enquiries so if you know him, please come forward as soon as possible.”
How to conact Merseyside Police
You can pass on any information via Twitter direct message @MerPolCC, ‘Merseyside Police Contact Centre’ on Facebook or Crimestoppers anonymously on 0800 555 111 with reference 22000726017.
To report a sexual offence allegation, call 101 where you will be spoken to by specially trained officers, or you can pass information to Crimestoppers anonymously on 0800 555 111.