The Red Ensign is a flag flown as an ensign by merchant ships and other private vessels of the United Kingdom. The red ensign has a Union Jack in the canton and may be defaced by a badge or shield in the fly. For more information see British ensigns.

The term is often used in particular to refer to the Canadian Red Ensign, the former de facto national flag of Canada. From 1892 it was the official flag for use on Canadian merchant ships, but on land the official national flag was the Union Jack. Despite its lack of official status the Red Ensign was widely used on land as well. In 1924 it was approved for use on Canadian government buildings outside Canada, and in 1945 for those inside Canada as well. The Red Ensign served until 1965 when it was replaced by today's maple-leaf flag. The flag bore various forms of the shield from the Canadian coat of arms in its fly during the period of its use. The picture shows the official form between 1957 and 1965.

Two Canadian provincial flags are Red Ensigns, the flag of Ontario and the flag of Manitoba.