Toronto police say the woman had flagged down the officer to get some information from him, at about 11 p.m. Wednesday.
The officer tripped when getting out of the car and the vehicle started rolling down Tranby Avenue, in the Avenue Road and Davenport Road area.
Police say the woman jumped into the car and brought it to a halt.
The vehicle crushed the officer's foot, as it rolled by. The woman stayed with him until paramedics arrived.
The officer is recuperating at home.