PC Farhan Ghadiali, 44, was found guilty of sexually assaulting the child while off duty at a party in Essex.
The incident took place on September 22, 2019. Following an investigation by Essex Police he faced a trial at Chelmsford Crown Court in May 2022.
The first trial resulted in a hung jury but a subsequent trial that concluded on March 24 found him guilty.
PC Ghadiali, of the Central West Command Unit, which covers Kensington and Chelsea, Westminster and Hammersmith and Fulham, has been suspended from duty since December 2020.
A misconduct hearing on Friday (May 19) found his actions amounted to gross misconduct and he was dismissed without notice.
Chief Superintendent Louise Puddefoot, of the Central West Command Unit, said: “PC Ghadiali was found guilty of an extremely serious offence.
“If the Met is to secure both the confidence and trust of the public we need to better protect our integrity by rooting out those that are not fit to serve, such as this individual who today has been dismissed.
“We are putting more resources into investigating allegations of such misconduct.”
Ghadiali, of Fyfield Road, Woodford Green, will be sentenced on Wednesday (May 24).
Detective Sergeant Sarah Barker, who led Essex Police’s investigation, said: “This has been a complex and sensitive investigation, and I can’t begin to imagine the turmoil the victim and their family have been through over the past four years.
“I’d like to thank them for placing their trust and confidence in us throughout this investigation, so that we could get them the justice they so rightfully deserves.
“Reports and offences of this kind never become less shocking and it’s as difficult as ever to accept that any police officer could do this to someone who looks up to them for protection.
“Nobody is above the law, and I hope that results like these show our determination to root out criminals, no matter who they are.
“No child or parent should ever have to experience this, and I hope that Ghadiali’s conviction will bring some comfort.”