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. Familiars often perform domestic duties and help in farming, but also aid the person in bewitching people. These spirits are also said to be able to inspire artists and writers (compare with muses).

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 in Steven Brust's Jhereg 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). The form taken by the familiar is also influenced by the personality of its master.

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