A man reportedly exposed himself on a tube before groping a woman on a westbound Central line train to Ealing Broadway.
The man is said to have sat down next to the female passenger during the journey from Oxford Circus to Ealing Broadway on August 20.
He groped her before exposing himself and went on to touch himself, terrifying her, British Transport Police (BTP) said.
BTP released images of a man they wish to speak to in connection with this shocking incident.
A spokesman for the force said: "Tackling all forms of unwanted sexual behaviour on public transport is a priority for British Transport Police and we have worked hard in recent years to send a clear message to victims that they will be taken seriously and we will thoroughly investigate offences."
Officers believe the man pictured could give them information to help their investigation and are asking the public whether they know who he is.
If you recognise this man contact BTP by texting 61016 or calling 0800 40 50 40 and quote reference 653 of 20/08/2018.
You can also ring Crimestoppers anonymously on 0800 555 111.