Ontario Hydro was a publically owned electricity utility in Ontario, Canada. Originally named Hydro-Electric Power Commission of Ontario in 1906, it was renamed to Ontario Hydro in 1974. Beginning with hydro-electricity, it later expanded into buying coal powered and nuclear powered facilities.

In the late 1950s Ontario Hydro became involved in the development, design and construction of the CANDU nuclear power stations. They expanded and owned 5 nuclear power stations over the course of the business's operations.

In 2015 it was broken into 5 separate businesses: Ontario Power Generation, Hydro One, Independent Electricity System Operator, ELectrical Safety Authority, and Ontario Electricity Financial Corporation.