In witchcraft, a familiar spirit, commonly called familiar (from Middle English familiar, related to family) is a spirit who obeys a witch, conjurer, etc., and serves and helps that person. Although they may not be as intelligent as their masters, they are often as intelligent as the average human. Familiars often perform domestic duties and help in farming, but also aid the person in bewitching people. If they look like ordinary animals, they can be used to spy on their masters' enemies. These spirits are also said to be able to inspire artists and writers (compare with muses). Some reclusive wizards rely on familiars as their closest friends.

Familiars were mentioned in Shakespeare's Macbeth, as the witches called their familiars. Many other works have utilized familiars. The most common species identified as familiars are cats (particularly black cats), owls, and sometimes frogs or toads.

Familiars are generally animals and such beings. They usually have some magical power, or are simply there to advance the story. In many modern fantasy stories, a magician's familiar is a magical creature, such as Vlad's flying lizardlike familiar, Loiosh, in Steven Brust's Vlad Taltos series. Furthermore, in many cases the power of the familiar is directly proportional to the power of the wizard (as in the His Dark Materials trilogy by Philip Pullman, although the comparison is not perfect, the nature of Pullman's daemons being dissimilar from standard notions of a familiar). The form taken by the familiar is also influenced by the personality of its master. The relationships between familiars and their masters vary by story. Some familiars do not have free will and are nothing more than tools of their masters while others are willing servants who can make their own decisions and would leave their masters if mistreated.

Familiars have appeared in several fantasy role playing systems, most notably as the companions of wizards and sorcerers in recent versions of Dungeons and Dragons.