British Transport Police have said that the incident took place on March 20 on the 10pm train from Bristol Temple Meads to London Paddington.
A man reportedly moved seats to be opposite the victim, before starting to masturbate while looking at her and smiling.
Officers have now released the image of a man they believe may have information that could help with their investigation.
Anyone who recognises the man or has any information about the incident has been asked to contact officers by texting 61016, or by calling 0800 40 50 40 quoting reference 529 of 21 March.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
