Portneuf County is an historic county in Quebec, Canada west of Quebec City on the Saint Lawrence River in Canada.
The county seat was Cap-Santé. Previously named Hampshire County formed in 1792 at the end of the French Regime and as a constituent ridding for the Legislative Assembly of Lower Canada before being renamed Portneuf County by a law of the province of Lower Canada (9 Georges IV ch. LXXII). It was bounded on the west by Champlain County on the northeast by Quebec County, and on the south by Lotbiniere County. Portneuf County consisted of six former seigneuries.
In the early 1980s Quebec abolished its counties and most of Portneuf County became the Portneuf Regional County Municipality. The southeastern part of the county was transferred to La Jacques-Cartier Regional County Municipality and the Quebec Urban Community (now Quebec City) while parts of western Portneuf County was transferred to Mékinac Regional County Municipality.