PC Thomas Lennon appeared at Westminster Magistrates' Court on Friday (May 25) where he was found guilty of the offence which happened at West Brompton station on July 10 last year.
The officer, who was off-duty at the time, assaulted another man and left him with minor injuries.
At the hearing on Friday, it was decided Lennon was not acting in self-defence and that his use of force was not proportionate or necessary in the circumstances.
Lennon, who has continued performing restricted duties with Metropolitan Police, has now been bailed to appear before Westminster Magistrates' Court on June 8 for sentencing.
Misconduct proceedings will be considered after his sentencing.