The airline says Toronto-to-London flight AC868 diverted to Shannon, Ireland, on Saturday after the child suffered a medical problem.
Air Canada says a doctor and a nurse were on board the plane and helped the flight crew to assist the child.
Emergency crews met the aircraft when it landed in Ireland, but Air Canada says local medical authorities pronounced the girl dead.
The Boeing 787 with 230 passengers on board continued on to London.
The Irish Sun reported that the coroner's office has been informed and a post-mortem examination will be carried out.
Neither Air Canada nor the Canadian government could immediately confirm the girl's nationality.