The 14-year-old was involved in an altercation with her attacker during a train journey and was assaulted by him as she left the train at Aberdare.
She suffered a bite mark on her forehead and bruising to her face.
British Transport Police said it was a "frightening experience".
They have been unable to locate the man in the picture and are appealing for help.
PC Nigel Maggs, the investigating officer, said: "This was a nasty incident which left the victim injured and in some distress.
"It was a frightening experience for the girl, who did not know her attacker.
"Behaviour such as this is completely unacceptable and will not be tolerated on the rail network.
"It is essential that we identify the person responsible for this attack, and I urge anyone with any information to contact police."
The incident happened at around 16:40 BST on Saturday, 2 June.
CCTV image released by police British Transport Police say the man had a Mohican-style haircut
Police said they believed the man in the picture had information which could help the investigation.