The incident happened around 2.15pm on a Bakerloo Line train to Waterloo.
A man exposed himself to a woman and then left the train at Waterloo Underground station.
Officers have released an image of a man they think could help with their investigation.
A British Transport Police spokesperson said: “Do you recognise this man?
“Officers investigating after a man exposed himself on board a train at Waterloo have released these images in connection.
“At around 4.15pm on Friday 22 August, a man on a Bakerloo Line train to Waterloo exposed himself to a woman.
“He then left the train at Waterloo Underground station.
“Officers would like to speak to the man pictured as they believe he may have information that could help their investigation.
“Anyone who recognises him is asked to contact BTP by texting 61016 or by calling 0800 40 50 40, quoting reference 435 of 22 August.
“You can also contact Crimestoppers anonymously on 0800 555 111.”
