The Blue Ensign is a flag, one of several British ensigns, used by certain organisations or territories associated or formerly associated with the United Kingdom. It is used either plain or defaced with a badge or other emblem.

In British maritime law and custom, an ensign is the identifying flag flown to designate a British ship, either military or civilian. Such flags display the United Kingdom Union Flag in the canton, with either a red, white or blue field, dependent on whether the vessel is civilian, naval, or in a special category. These are known as the red, white, and blue ensigns respectively.