The passenger got on board the Underground train at Hounslow Central between 8.20am and 8.40am.
He asked a man already on the train to move bags from a seat and a buggy blocking the carriage.
The man refused to do so and punched the victim in his face, before getting off at Knightsbridge.
The suspect is described as black with dreadlock hair and wearing a grey tracksuit.
He was with a woman with a buggy and a suitcase.
Anyone with information or who may have seen the incident is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference number 116 of March 31.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
