The incident took place between London Bridge and Greenwich at around 6.15am on October 10.
Officers say the man was seen touching himself while looking at women on the train.
They have released an image of a man they want to speak to as they believe he may have information that could help their investigation.
The British Transport Police are asking anyone who recognises him to contact them by texting 61016 or calling 0800 40 50 40, quoting reference 92 of October 10.
Alternatively, members of the public can contact Crimestoppers anonymously on 0800 555 111.
