The Familiar of Zero (Japanese: ゼロの使い魔, Hepburn: Zero no Tsukaima) is a Japanese fantasy light novel series written by Noboru Yamaguchi, with illustrations by Eiji Usatsuka. Media Factory published 20 volumes between June 2004 and February 2011. The series was left unfinished due to the author's death in 2013, but was later concluded in two volumes released in February 2016 and February 2017 with a different author, making use of notes left behind by Yamaguchi. The story features several characters from the second year class of a magic academy in a fictional magical world with the main characters being the inept mage Louise and her familiar from Earth, Saito Hiraga.

Between 2006 and 2012, the series was adapted by J.C.Staff into four anime television series and an additional original video animation episode. The first anime series was licensed by Geneon Entertainment in English, but the license expired in 2011. Sentai Filmworks has since re-licensed and re-released the first series and released the other three series in North America. A manga adaptation illustrated by Nana Mochizuki was serialized in Media Factory's manga magazine Monthly Comic Alive between June 2006 and October 2009. A sequel manga was serialized from January 2010 to March 2013. Both manga were released by Seven Seas Entertainment in North America. Two additional spin-off manga were also created, as were three visual novels.

The nobles of Halkeginia are magicians known as "mages" or "magi", and must use a wand or a staff to cast spells. Not all magi are nobles, as some have been forcibly demoted or given up their nobility and use their magic for thievery or to work as mercenaries.

There are four primary magic elements, corresponding to the four Classical elements: Fire, Water, Air, and Earth. Magi have an affinity to one of those elements. There is a fifth element, the legendary lost branch of element, known as Void. Magi can combine different magical elements or even stack the same element on top of itself to make spells more powerful. The power of a mage is determined by how many elements he or she can combine. There are four ranks of magician based upon this factor. Those who cannot combine any element together have the first rank, which is symbolized as a dot. Most magic students are in this class. The ability to combine two, three or four elements together leads to a magic user being referred to as a line, triangle, or square mage.

One of the major themes is that humans and technology from Earth displaced into the world of Halkeginia are considered to be highly magical, since Halkeginia itself has relatively primitive science and technology. For example, a magical item called the "Staff of Destruction" is, in reality, a human-made Vietnam-era M72 LAW rocket launcher. Other examples of Earth technology, often weaponry, appear throughout the story, such as a World War II era Mitsubishi A6M Zero fighter plane.

Louise Françoise Le Blanc de La Vallière is a member of the nobility who is terrible at magic, as her attempts usually result in an explosion. She is nicknamed "Louise the Zero" by her classmates, due to her inability to use any of the four magic elements. Early in the school year at the Tristain Academy of Magic, the second year students perform a special ritual where they summon their familiar, which serves as their eternal companion, protector and partner, and is usually some sort of magical creature. But Louise summons Saito Hiraga, an ordinary teenage boy from Japan, leaving her totally humiliated.

Due to the sacredness of the ritual (which makes it blasphemous to do it more than once), Louise reluctantly accepts Saito as her familiar, but proceeds to treat Saito as any other familiar, only worse: making him wash her clothes and sleep on a bed of hay, and whipping him with a riding crop whenever he upsets her. The Familiar of Zero follows the adventures of Louise and Saito as they help their classmates and friends, while occasionally blundering into situations where they risk their lives to save one another and Tristain. Saito tries to find a way to get back to Japan, but he also gains a mysterious power that allows him to wield swords and other weapons to perform heroic feats. They also eventually learn the truth behind Louise's magic (in)abilities. As Saito's and Louise's friendship grows and they start to work together, they ultimately fall in love with each other.

The Familiar of Zero began as a series of light novels written by Noboru Yamaguchi and illustrated by Eiji Usatsuka under the original title Zero no Tsukaima. Media Factory published 20 volumes in Japan between June 25, 2004 and February 25, 2011. Further publication was threatened by Yamaguchi's advanced cancer, which he died from on April 4, 2013, leaving the series unfinished. Yamaguchi had planned the series to end with 22 volumes. However, it was later decided by the publisher to continue the series in the author's absence, due in part to calls from fans to continue the series, as well as Yamaguchi's family wanting to see it completed. Yamaguchi had dictated the remaining plot of the series, including its ending, and entrusted it to the editors. Media Factory released volume 21 on February 25, 2016 and the final volume 22 on February 24, 2017. Seven Seas Entertainment licensed the series in 2007 under the title Zero's Familiar, but later cancelled the release of the novels before releasing any.^{[citation needed]}