In Warhammer 40,000, the Imperial Fists are one of the first founding chapters of Space Marines. They stand out from other space marines due to having no fixed home world, although they are most frequently based on Earth, using instead the 10,000 year old mobile space fortress Phalanx as their fortress monastery. They maintain recruitment-chapels on various worlds spread throughout the Imperium. Part of their duties during the Great Crusade was to function as the emperor's "personal praetorians", accompanying him everywhere and usually used to strike a decisive blow against the enemy. The role of the Emperor's bodyguard has since been taken over by the Adeptus Custodes.

Their armour is a golden yellow, with trim according to codex company colours. Their Primarch was named Rogal Dorn and is usually regarded as having been the most level headed and fair of the Primarchs, although in the view of the Iron Warriors he was arrogant and spiteful. The Imperial Fists were one of the three chapters who defended the Emperor's Palace during the Horus Heresy, in fact, it was only due to the intervention and self-sacrifice of a squad of Imperial Fist terminators that the Emperor could strike down Horus. It was Rogal Dorn who discovered the bodies of the Emperor, Sanguinius and Horus, receiving the last words of the Emperor instructing him to build the Golden Throne which sustains him to this day.

The Imperial Fists, like the Iron Warriors, were the siege masters of the original founding chapters and as such rely greatly on artillery and heavy weapons, although not to the extent of their traitorous former brethren. Their shared specialisation led to great rivalry and eventually hatred between the two chapters and their Primarchs, and after the Heresy the Imperial Fists continued to pursue the Iron Warriors for some time, culminating in the incident of the "Iron Cage" - which essentially ended in a stalemate broken by the arrival of the Ultramarines chapter.

After the Heresy, the Imperial Fists were broken up, as were the other chapters, and their more notable successor chapters include the Crimson Fists and the Black Templars. The Imperial Fists, however, were among the slowest of the Chapters to co-operate with the tenets of the Codex Astartes written by the Primarch of the Ultramarines, Roboute Guilleman.

The Imperial Fists' geneseed has been somewhat corrupted over the millennia, so two traits have emerged. One is the loss of two of the special organs produced by Space Marines: the Betcher's Gland, which allows the Marine to produce poisonous/acidic spittle, and the Sus-an Membrane, which allows a Marine to enter a state of suspended animation.

The second is the Imperial Fists' over-zealous self-castigation. They often make use of a device called the "pain glove", which encases the whole body and stimulates pain neurons. The Imperial Fists constantly feel the need to punish themselves for the smallest inadequacy, failure or infraction. The last remaining relic of Rogal Dorn, his skeletal hand, is displayed in their fortress monastery as a reminder of the ultimate dedication required of the Space Marines.

Although the Imperial Fists have a preference for long range engagement and specialise in siege warfare, they are also well known for their passion for duelling, a trait inherited from the men of Earth from whom the originally warriors of the legion were recruited, and many can display scars from this time honoured practice.