The incident happened at approximately 8.50am on Sunday 7 January on a northbound Bank branch service. The incident happened between Euston and Camden Town.
The victim was seated when a man touched her inappropriately. As the train arrived at Camden, he then stood in front of the victim and exposed himself. The victim then ran off the train onto a different service.
The offender then boarded a separate northbound train heading towards Edgware.
Anyone with information is asked to contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 118 of 07/01/18. Alternatively, pass information anonymously to Crimestoppers on 0800 555 111.