At around 3,30pm on Friday, January 5, a 16-year-old girl boarded the Piccadilly Line at Kings Cross.
She noticed a man staring at her and then realised he was touching himself over his trousers, British Transport Police said.
He then left the Tube at Southgate underground station.
Investigating officers have now released an image of a man they believe may have information that could help their investigation.
Anyone who recognises the man, or who has information which could help the investigation is asked to contact British Transport Police.
You can do this by texting 61016 or calling 0800 40 50 40 with the reference 2400002273.
Alternatively, you can contact Crimestoppers anonymously by calling 0800 555 111.