According to the British Transport Police (BTP), the man was on a train between London Bridge and Greenwich on October 10 at 6.51am, as he allegedly touched himself.
It is said he was seen looking at woman while on board.
An image has been released of a man who police would like to speak to, as officers hope he may have information which could help their investigation.
The man pictured is seen with a beard and wearing a yellow cap.
Anyone who recognises him is asked to contact BTP by texting 61016 or calling 0800 40 50 40, quoting reference 92 of 10 October.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.
