The flag of Saint Helena, Ascension and Tristan da Cunha is a British Blue Ensign with an aspect ratio of 1:2. It features a blue field (Pantone 280C) with the Union Jack in the canton and the territory's coat of arms on the fly side.

The coat of arms consists of a shield divided into three sections, each representing one of the islands. The top left section has a green field (Pantone 355C) with a golden-yellow (Pantone 116C) representation of Saint Helena's plover. The top right section has a black field (Pantone Black C) with a green sea turtle on it. The bottom section has a blue field (Pantone 300C) with a yellow Tristan da Cunha rock lobster. Above the shield, there's a sailing ship, and below it, there's a white scroll with the territory's motto "Our faith is our strength" written in black.

Exact geometric properties and measurements are not standardized for this flag, as they can vary depending on the manufacturer or specific use case.