She was travelling on the 393 bus towards Kentish Town on May 7 when she turned around and saw that the passenger behind her had pulled down his trousers and was exposing himself.
The man then got off the bus at Goodinge Health Centre in North Road, Islington, just after 8am.
Police have released a CCTV image of a man they wish to speak to about this incident.
“Detectives from the Met’s Roads and Transport Policing Command are investigating and have released a CCTV image of a man who they wish to urgently identify and speak to about this incident,” a spokesman said.
“He is described as white, approximately 6ft tall, with a large build and a scruffy appearance.
“At the time of the incident he was wearing a striped top and dark trousers and was carrying a dark coloured holdall bag.”
Anyone who recognises the man is asked to call 101 or tweet @MetCC providing the reference 1521/07MAY. Information can also be provided to Crimestoppers, anonymously, by calling 0800 555 111.