A white stag or white deer is a red deer with a condition known as leucism that causes its hair and skin to lose its natural colour. The white stag has played a prominent role in many cultures' mythology.

White deer hold a place in the mythology of many cultures. The Celtic people considered them to be messengers from the otherworld; it also played an important role in other pre-Indo-European cultures, especially in the north. The Celts believed that the white stag would appear when one was transgressing a taboo, such as when Pwyll tresspassed into Arawn's hunting grounds. Arthurian legend states that the creature has a perennial ability to evade capture; and that the pursuit of the animal represents mankind's spiritual quest. It also signalled that the time was nigh for the knights of the kingdom to pursue a quest.

In Christianity, the white stag was partly responsible for the conversion of the martyr Saint Eustace. Eustace saw a vision of Christ between the stag's antlers and was told that he would suffer for Christ.

The white stag has also been invoked in contemporary society for its symbolism. Robert Baden-Powell, founder of the Scouting movement, spoke to Scouts at the 1933 World Jamboree in Gödöllő, Hungary, about the white stag:

The White Stag has a message for you. Hunters of old pursued the miraculous stag, not because they expected to kill it, but because it led them in the joy of the chase to new and fresh adventures, and so to capture happiness. You may look on the White Stag as the true spirit of Scouting, springing forward and upward, ever leading you onward to leap over difficulties, to face new adventures in your active pursuit of the higher aims of Scouting.

—Baden-Powell's farewell speech to the Scouts

Today, the Hungarian branch of Scouts uses the white stag as a symbol. The white stag is also prevalent in Hungarian mythology; it was believed that a white stag led the brothers Hunor and Magor to Scythia, an action which preceded the formation of the Hun andMagyar people. The White Stag Leadership Development Program bases much of its spirit and traditions on the white stag of Hungarian mythology.

The colour white has long been associated with purity; and in Celtic culture, the colour also represented the otherworld. In modern society, people have acted to protect the white stag as a vestige of beauty, and the hunting of the white stag has often met with anger, because of its rare and elusive nature. White can also mean a symbol of peace or a truce.

For early man, the deer resprented a valuable resource, providing nourishment, clothing and other accessories; and the deer may have played a role in totemic culture.