The Thousand Sons are one of the legions of Chaos Space Marines in the game Warhammer 40,000. The main feature that distinguishes them from other legions is the amount of sorcerers they use in combat and their preference to stay away from close combat.

After a crushing defeat by the Space Wolves during the Horus Heresy, the Thousand Sons allied with Horus and swore loyalty to Tzeench, Chaos God of Change. Soon after, the Sons started suffering debilitating mutations from their Chaos patron. The Chief Librarian of the Thousand Sons, Ahriman, devised a way to prevent further mutation, and in a powerful spell known as the Rubric of Ahriman, he disintegrated the bodies of the Thousand Sons, trapping their essence in their armour.

Ahriman's cabal was exiled to the Planet of Sorcery for his action, and now they seek out magical artefacts and talented psykers; their threat is so great that the Inquisition will sooner destroy a precious artefact than allow it to fall into Ahriman's hands.

The Thousand Sons' pre-heresy color scheme was red, in recognition of their primarch Magnus The Red, and later became Royal Blue with gold or yellow trim. Many Thousand Sons marines wear ornate crests on their helmets similar to the Burial mask of Tutankhamun. The symbol that Thousand Sons marines bear upon their shoulder pads is the Ouroboros, but used to be a red 'M', for Magnus. 'M' is also the roman numeral for one thousand, and this is what they draw their chapter name from.