Warhammer 40,000 is a miniature wargame produced by Games Workshop. It is the most popular miniature wargame in the world, and is particularly popular in the United Kingdom. The first edition of the rulebook was published in September 1987, and the ninth and current edition was released in July 2020.

As in other miniature wargames, players enact battles using miniature models of warriors and fighting vehicles. The playing area is a tabletop model of a battlefield, comprising models of buildings, hills, trees, and other terrain features. Each player takes turns moving their model warriors around the battlefield and fighting their opponent's warriors. These fights are resolved using dice and simple arithmetic.

Warhammer 40,000 is set in the distant future, where a stagnant and dogmatic human civilization is beset on all sides by hostile aliens and supernatural creatures. The models in the game are a mixture of humans, aliens, and supernatural monsters, wielding futuristic weaponry and supernatural powers. The fictional setting of the game has been developed through a large body of novels, published by Black Library (Games Workshop's publishing division). Warhammer 40,000 took its name from Warhammer Fantasy Battle, which is a medieval fantasy wargame also produced by Games Workshop. Warhammer 40,000 was initially conceived as a science fiction counterpart to Warhammer Fantasy, and while they are not connected to each other in a shared universe, their settings share similar themes.

Warhammer 40,000 has spawned a large amount of spin-off media. These include board games such as Space Hulk, which is about combat in the narrow corridors of derelict spacecraft, and Battlefleet Gothic which simulates spaceship combat. Video game spin-offs, such as the Dawn of War series, have also been released.