Detectives from the Roads and Transport Policing Command are appealing for information following an exposure committed onboard a bus travelling between Cromwell Bus Station and Putney Bridge.
The exposure occurred on a Route 85 bus, at approximately 21:05hrs on Monday, 22 January. After the offence, the suspect got off the bus at a stop near Asda in Roehampton.
The suspect is described as an Eastern European man, 5ft 8ins tall with shaved receding dark hair and dark facial stubble. He was wearing a black jacket with a logo on the back of it and dark coloured jeans; he was also carrying a rucksack.
Detective Constable Clare Beauchamp, of the Roads and Transport Policing Command said: "The Met will not tolerate any form of sexual incidents and we strongly urge anyone who has been a victim of this crime to call police and report it.
"If you know who this man is, or can assist our investigation, please call 101 and quote reference 8111/22Jan18."
To give information anonymously, call Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org