British Transport Police (BTP) said the incident happened sometime between 9.50am and 10am on Monday, May 13.
"Detectives investigating a report of sexual harassment on board a train to Charing Cross are today releasing [a CCTV] image in connection,” a BTP statement read.
"Sometime between 9.50am and 10am on Monday, May 13, the victim, a schoolgirl, boarded the 9.52am West St Leonards service to Charing Cross.
"A man in the carriage, who had boarded the service at Hastings, moved seats to sit closer to the girl before taking out his phone and directing it at her.”
The man left the train at Charing Cross, police said.
A spokesperson added: “Officers believe the man in the CCTV image may have information that could help their investigation.
"Anyone who recognises him, or has any other information, is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference 228 of May 13. Alternatively, you can call Crimestoppers anonymously on 0800 555 111.”
1. Police investigate report of sexual harassment on train
Officers believe the man in this CCTV image may have information that could help their investigation. Photo: British Transport Police