The incident happened on Sunday, 20 February at about 7.30pm
The victim, a girl in her mid-teens, had boarded a number 99 bus at Woolwich DLR station.
A man, who was not known to the victim, got on the bus at the same time as the girl and both went to the upper deck.
As the bus travelled towards Bexleyheath the man engaged the girl in conversation and is then alleged to have carried out a sexual assault by inappropriate touching.
The man is described as aged 18-25, 5ft 10in and of Asian appearance.
After the assault the man got off the bus at Plumstead Manor Way.
Any witnesses or anyone with any information is asked to call police on 101 or contact via Twitter @MetCC. Please quote CAD 1837/22FEB
To give information anonymously contact Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org.
