The man touched the 15-year-old boy as the teenager was attempting to board a train at Sheffield railway station on Thursday, March 29.
CRIME: Man seriously injured after ANOTHER stabbing in SheffieldAn investigation has been launched by British Transport Police.
POLICE: Man stabbed in Doncaster town centreCOURT: Suspended sentence for man who left victim with bleed on the brain in South Yorkshire pub attackA BTP spokesman said: "Officers are appealing for witnesses to a sexual assault of a teenager at Sheffield station.
"The 15-year-old was boarding the 4.40pm train to Norwich on March 29 when he was sexually touched by a man.
"He confronted the offender, who was standing directly behind him, as several other passengers also tried to get on the train.
"Did you see this incident?"
The man officers want to trace is in his fifties and has shoulder length, thin, grey hair.
He was wearing a brown parka style coat and brown jeans or trousers at the time.
Anyone with information should call BTP on 0800 40 50 40, quoting 217 of April 18.
Crimestoppers can be contacted on 0800 555 111.