A Certain Magical Index (Japanese: とある魔術の禁書目録（インデックス）, Hepburn: Toaru Majutsu no Indekkusu) is a Japanese light novel series written by Kazuma Kamachi and illustrated by Kiyotaka Haimura, which has been published by ASCII Media Works under their Dengeki Bunko imprint since April 2004 in a total of three separate series. The first ran from April 2004 to October 2010, the second from March 2011 to July 2019, and the third from February 2020 to present.

The plot is set in a world where supernatural abilities exist. The light novels focus on Toma Kamijo, a young high school student in Academy City with an unusual ability in his right hand as he encounters an English nun named Index. His ability, which allows him to cancel other powers by touching them, and his relationship with Index prove dangerous to other sorcerers and espers who wanted to discover the secrets behind him and Index, as well as the city.

A manga adaptation by Chuya Kogino began serialization in Monthly Shōnen Gangan in April 2007. J.C.Staff produced two 24-episode anime series between 2008 and 2011. An animated film was released in February 2013. A 26-episode third season aired between 2018 and 2019. Several spin-offs and other adaptations have also been made, including several video games.

After being rejected for the Dengeki Novel Prize, Kamachi was contacted by Kazuma Miki, an editor at ASCII Media Works who had him write several test novels. He got the chance to write a full series and decided to create it with the concept of exploring the rules of magic, rather than just having it exist, and the inclusion of science to oppose its elements. The resulting series has seen success with both critics and audiences, with critics praising the action and characters.