The suspect got off at Kings Cross station
The 38-year-old said he was standing on the busy train from Finsbury Park when a man positioned himself very close to him and began to press against him and touch him in appropriately.
The “prolonged” attack continued until the suspect got off the Tube at Kings Cross.
Investigating officer, Det Con Suleman Yazdani from British Transport Police, said: “This was a disgraceful and prolonged assault committed on a busy commuter tube service.
Witnesses are urged to contact the police
This was a disgraceful and prolonged assault committed on a busy commuter tube service
Investigating officer, Det Con Suleman Yazdan
"As you can imagine this was a frightening experience for him as he quite rightly did not expect or want this man to touch him.”
The suspect has been described as a man of slender build and around 5ft 7in tall.
He is believed to be in his mid-twenties, with blue eyes and blonde hair which was curly on top.
He was wearing black jeans, trainers, a camouflage jacket and was holding a bright rucksack at the time of the alleged attack.
The suspect also had a black earring in his left ear.
Detective constable Suleman Yazdani, added: "If you know someone who matches the description then please get in touch as your information could be vitally important.
“Unwanted sexual behaviour such as this will never be tolerated and we work closely with our partners throughout the capital to encourage victims to report offences.
"Recently, we launched a refreshed version of the highly successful Report it to Stop it campaign, to tackle under-reporting and raise awareness of the reporting services.”
Anyone with information relating to the incident which took place at about 7.30am on Wednesday 22 March can call BTP on 0800 40 50 40, or text 61016 quoting 113 of 23/03/2017 as a reference, or Crimestoppers anonymously on 0800 555 111.