The Diocese of Peterborough (Latin: Dioecesis Peterboroughensis) is a Latin Church ecclesiastical territory or diocese of the Catholic Church in Ontario, Canada.

It is a suffragan in the ecclesiastical province of the metropolitan Archdiocese of Kingston, Ontario.

[1] Established on 25 January 1874 as Apostolic Vicariate of Northern Canada, on territory split off from the Diocese of Kingston in Ontario, the Diocese of Peterborough was first established July 11, 1882 by Pope Leo XIII.

[2] As per 2014, it pastorally served 61,700 Catholics (13.8% of 447,000 total) on 25,900 km2 in 40 parishes and a mission with 69 priests (56 diocesan, 13 religious), 8 deacons, 92 lay religious (13 brothers, 79 sisters) and 2 seminarians.

The Diocese of Peterborough includes the Muskoka and Parry Sound Districts, the counties of Peterborough, Northumberland, and Victoria, that portion of the Regional District of Durham which formally was the County of Durham and five townships of the County of Haliburton.