The flag of Ontario is the provincial flag of Ontario, Canada. It is a defaced Red Ensign, with the Royal Union Flag in the canton and the Ontario shield of arms in the fly. The flag of Ontario was derived from the Canadian Red Ensign, which was used as a civil ensign and as a de facto flag of Canada from the late 19th century to 1965. It was adopted in a period when many Canadian provinces adopted their own flags. May 21 is Ontario Flag Day.

The Red Ensign or "Red Duster" is the civil ensign of the United Kingdom of Great Britain and Northern Ireland. It is one of the British ensigns, and it is used either plain or defaced with either a badge or a charge, mostly in the right half.