The Australian red ensign is the civil ensign of Australia, the flag of nationality flown by Australian registered ships. It is a red version of the national flag, which is mainly blue. Both flags resulted from the Commonwealth Government's 1901 Federal Flag Design Competition which required two entries: an ensign for Commonwealth Government use and another for the merchant navy. The winning design for the merchant ensign was based on the traditional British red ensign and featured the Southern Cross and Commonwealth Star.

The flag of Australia is based on the British Blue Ensign—a blue field with the Union Jack in the upper hoist quarter—augmented with a large white seven-pointed star and a representation of the Southern Cross constellation, made up of five white stars. Australia also has a number of other official flags representing its people and government bodies.