Renfrew County is a county and census division in the Canadian province of Ontario. It straddles the west bank of the Ottawa River. There are 17 municipalities in the county. Its county seat is Pembroke, which is geographically within the county but administered independently.

When Carleton County was withdrawn from Bathurst District in 1838, Renfrew County was severed from part of the remaining Lanark County, but the two remained united for electoral purposes. By 1845, all lands in the District had been surveyed into the following townships:

Counties comprising Bathurst District (1845)

Lanark	Renfrew
Bathurst Beckwith Drummond Dalhousie Darling North Elmsley North Burgess Levant Lanark Montague Ramsay North Sherbrooke South Sherbrooke	Admaston Blithefield Bagot Bromley Horton McNab Pakenham Pembroke Ross Stafford Westmeath

Effective January 1, 1850, Bathurst District was abolished, and the "United Counties of Lanark and Renfrew" replaced it for municipal and judicial purposes. The counties remained united for electoral purposes in the Parliament of the Province of Canada, referred to as the County of Lanark, until Renfrew gained its own seat in 1853.

The separation of Renfrew from Lanark began in 1861, with the creation of a Provisional Municipal Council that held its first meeting in June 1861. The United Counties were dissolved in August 1866.

The territory was originally described in 1838 as consisting of:

...the Townships of Packenham, McNab, Horton, Ross, Westmeath and Pembroke, together with the unsurveyed lands within the District of Bathurst, and all the islands in the Ottawa River, wholly or in greater part opposite to the said Townships and unsurveyed lands...

In 1851, Pakenham was transferred to Lanark, while Renfrew was expanded through the addition of:

...all that tract of land lying between the Western Boundaries of the Townships of Lavant, Blithfield, Admaston, Bromley, Stafford and Pembroke and the Ottawa River, and a line drawn parallel to the general course of the said Boundaries of the said Townships from the western corner of the Township of Clarendon to the Ottawa River.

In 1860, the newly surveyed townships of Miller and Canonto were transferred to Frontenac County, while the townships of Raglan, Lyndoch, Radcliffe and Brudenell were added to Renfrew. The final expansion of the County occurred in 1877-1878, with the transfer of the United Townships of Hagarty, Sherwood, Jones, Richards and Burns, and the United Townships of Head, Clara and Maria, from Nipissing District.

Once all lands had been surveyed in 1878, the County consisted of the following geographic townships: