A berserker is a member of an unruly warrior gang that worshipped Odin, the supreme Norse deity, and attached themselves to royal and noble courts as bodyguards and shock troops. They were known for their savagery in battle and their animal-skin attire, which contributed to the development of the werewolf legend in Europe. The term "berserker" comes from the Old Norse word "berserkr," which means "bearskin".

Berserkers were described as fearsome warriors who fought in a trance-like fury, a characteristic which later gave rise to the modern English word "berserk". They were attested to in numerous Old Norse sources and appeared prominently in a multitude of sagas and poems. Many earlier sagas portrayed berserkers as bodyguards, elite soldiers, and champions of kings, but this image would change as time passed and sagas would begin to describe berserkers as boasters rather than heroes, and as ravenous men who loot, plunder, and kill indiscriminately. Within the sagas, berserkers can be narrowed down to four different types.

The rage the berserker experienced was referred to as "berserkergang" (Berserk Fit/Frenzy or The Berserk movement). This condition was described as a trance-like state where the berserker experienced a loss of identity as a human. The sources frequently state that neither edged weapons nor fire affected the berserks, although they were not immune to clubs or other blunt instruments.

In popular culture, berserkers have been portrayed as fearsome enemies to meet, often said to be so intoxicated by battle-lust that they bit their shields, attacked boulders and trees, and even killed each other while they were waiting for battles to begin.