A pervert exposed himself to a woman on the DLR before leaving the train at Cutty Sark station, police say.
British Transport Police want to trace a man shown in these CCTV stills who may have vital information about the incident - which happened at 8.30pm on January 3.
Investigating officer PC David Udomhiaye said the 48-year-old victim was travelling from Canary Wharf when a man sat opposite her.
When the train reached Island Gardens the man exposed himself and carried out an indecent act, before leaving the service at Cutty Sark.
PC Udomhiaye said: "This was an intimidating and upsetting experience for the victim.
"We don’t tolerate unwanted sexual behaviour of any kind on the transport network and take all reports seriously. I’d urge anyone who recognises the man pictured to come forward and speak to officers investigating the incident."
Anyone who can help should police on 0800 40 50 40 or Crimestoppers on 0800 555 111.