The offence occurred on Friday, 7 July and detectives want to speak with 36-year-old Parminder Thindh in relation to this investigation.
The victim, a 12-year-old girl, boarded the Route 101 bus from Manor Park heading west at approximately 08:15hrs, and remained standing.
The suspect boarded the bus near Ruskin Avenue and stood next to the girl. As the bus became more crowded, he allegedly began to touch her without looking at her.
The victim hoped he would stop touching her, but when he didn't, she got off the bus a stop early and had to push his hand away as she did so.
Thindh is described as 5ft 8ins tall, of medium build and aged in his thirties.
Detective Constable Lee Warrington, from Child Abuse and Sexual Offences Command, said: "We would like to talk to the man pictured about this sexual assault which left the young victim extremely distressed. Sexual assaults are totally unforgivable and we will always investigate these types of offences thoroughly. If you know how we can reach Mr Thindh, we would like to hear from you."
If you recognise the man pictured, please contact the police on 101 or via Twitter @MetCC.
Alternatively, call Cirmestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org