Peterborough County is located in Southern Ontario, Canada. The county seat is The City of Peterborough, which is independent of the county.

Peterborough is a city on the Otonabee River in Ontario, Canada, about 125 kilometres northeast of Toronto. According to the 2021 Census, the population of the City of Peterborough was 83,651. The population of the Peterborough Census Metropolitan Area (CMA), which includes the surrounding Townships of Selwyn, Cavan Monaghan, Otonabee-South Monaghan, and Douro-Dummer, was 128,624 in 2021. In 2021, Peterborough ranked 32nd among the country's 41 census metropolitan areas according to the CMA in Canada. The current mayor of Peterborough is Jeff Leal.