The man, who is in his 20s, had fallen asleep on the London Underground night tube on Saturday.
He woke up at around 4am to another man sexually assaulting him.
The victim then pushed the man away and chased him off the service at Hainault station.
Units from British Transport Police and the Metropolitan Police attended the scene.
Detectives are now appealing for anyone who witnessed what happened or who may have been on an early morning Central Line train, to get in touch.
Likewise, a CCTV image has been released.
Investigating officer DC Andy Parkinson, said: ‘Understandably this was an appalling ordeal for this man and our specialist officers are providing him with support.
‘We will simply never tolerate any form of unwanted sexual behaviour and a number of our officers are now working to trace the offender.
‘As part of our enquiries, we are looking to speak with anyone who was on an eastbound Central Line service at this time and saw a man acting suspiciously.
‘In particular, I am keen to hear from anyone who saw a man running off in the direction of Thurlow Gardens shortly after 4am.
‘We are today releasing a CCTV image of a man we would like to speak with in connection to this assault. If you know who this man is then please get in touch as soon as possible.