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

The southern section of the county is mix of agriculture, urban and lakefront properties. The northern section of the county is mostly sparsely populated wilderness, with numerous rivers and lakes mostly within Kawartha Highlands Provincial Park.

The county contains the Kawarthas, a major tourist region.

In 1615, Samuel de Champlain was one of the first western explorers who traveled through the area, coming down from Lake Chemong and portaging down a trail, which is approximated by present-day Chemong Road, to the Otonabee River and stayed for a brief time near the present-day site of Bridgenorth, just north of Peterborough.

The area was initially part of Northumberland County, which was formed by proclamation of the first Lieutenant Governor of Upper Canada, John Graves Simcoe in 1792, and defined by statute in 1798. In 1802, Northumberland was included in the Newcastle District. In 1841, the northern part of the District was detached to form the Colborne District, with the northern portion of Northumberland county made into the new County of Peterborough. It consisted of the following territory:

The county was named in honor of Col. Peter Robinson, who in 1825 brought 2,000 settlers from Ireland. The route taken was by way of Port Hope, Rice Lake and the Otonabee River, the same route used by the first settlers that entered this region in 1818.

The centre of the County was originally the courthouse, which is still considered an important historical site.

In 1851, Peterborough County was divided into the counties of Peterborough and Victoria, which were united for municipal purposes as the United Counties of Peterborough and Victoria.