A Certain Magical Index is set in a world where supernatural abilities are a reality. Individuals who possess special powers acquired via science are called Espers (超能力者, Chōnōryokusha). Those Espers who gain their abilities without the aid of special scientific instruments, whether at birth, are referred to as Gemstones (原石, Genseki). Others, known as Magicians (魔術師, Majutsushi), gain their powers upon mastering the power of magic, either from obtaining knowledge from different mythologies or by using mystical artifacts, though the existence of magicians is a secret to the public. While Magicians align themselves with different beliefs, Espers are aligned with scientific institutions. This leads to a power struggle between the magic and science factions for control of the world.
The story is set in Academy City (学園都市, Gakuen Toshi), a technologically advanced city located in west of Tokyo that is known for its educational and research institutions. Tōma Kamijō is a student in Academy City whose right hand, the Imagine Breaker, has the ability to negate any supernatural power, but also negates his own luck, much to his chagrin. One day, Tōma meets a young English girl named Index – a nun from Necessarius, a secret magic branch of the Church of England, whose mind has been implanted with the Index Librorum Prohibitorum – 103,000 forbidden magical books that the Church has stored in secret locations. His encounter with her leads him to meet others from the secret worlds of science and magic. Tōma's unusual power places him at the center of conflicts between the Magicians and Espers in Academy City who try to unravel the secrets behind Academy City, Index, and Tōma's own special power.
Besides its own manga adaptation, the series also has two manga series which are side stories focusing on other main characters. One of them is A Certain Scientific Railgun, which focuses on Mikoto Misaka, an Electromaster and one of the most powerful Espers in the city. The second, A Certain Scientific Accelerator, focuses on Accelerator, a teenager who can control vectors, known as the most powerful Esper in Academy City.
A Certain Magical Index began as a light novel series written by Kazuma Kamachi, with illustrations by Kiyotaka Haimura. ASCII Media Works published 25 volumes between April 10, 2004 and August 10, 2011 under their Dengeki Bunko imprint; 22 comprise the main story, while the other three are short story collections. Yen Press has licensed the series in North America and began releasing it under their Yen On imprint in November 2014. A sequel series of light novels titled New Testament: A Certain Magical Index (新約 とある魔術の禁書目録, Shin'yaku Toaru Majutsu no Index) began publication on March 10, 2011; there have been 17 volumes published as of November 10, 2016.
The series has been adapted into two manga series. The one based on the novels is illustrated by Chuya Kogino and started serialization in the May 2007 issue of Square Enix's Monthly Shōnen Gangan. The first tankōbon volume was released on November 10, 2007 and as of February 22, 2014, 13 volumes have been published. Yen Press has licensed the series in North America. The manga is also published in Italy by Star Comics and in Germany by Carlsen Comics. A manga adaptation of the film Toaru Majutsu no Index: Endymion no Kiseki, illustrated by Ryōsuke Asakura, began serialization in the March 2013 issue of Monthly Shōnen Gangan.
A spin-off manga titled A Certain Scientific Accelerator (とある科学の一方通行, Toaru Kagaku no Akuserarēta) is illustrated by Arata Yamaji, and began serialization in the February 2014 issue of Dengeki Daioh sold on December 27, 2013. The first tankōbon volume was released on July 26, 2014.
A 24-episode anime adaptation of A Certain Magical Index was produced by J.C.Staff and directed by Hiroshi Nishikiori, and aired in Japan from October 4, 2008 to March 19, 2009. The anime was collected into eight DVD and Blu-ray Disc sets which were released from January 23 to August 21, 2009. In addition, four mini bonus episodes depicting Index super deformed, titled A Certain Magical Index-tan, were bundled with limited edition copies of the first and fifth DVDs of the first season, and the first BD/DVD release of the second season. The anime covered the first six novels in the series. Funimation has licensed the series in North America. An English language dub began streaming on Funimation's website in September 2012 and was released on DVD on December 11, 2012. The first season aired in North America on the Funimation Channel on January 21, 2013. A second season, titled A Certain Magical Index II, aired in Japan between October 8, 2010 and April 1, 2011, and was also streamed on Nico Nico Douga. The limited edition of the first BD/DVD volume of Index II, released on January 26, 2011, contained a A Certain Scientific Railgun mini-novel. The first season of the English dub was released in 2012 while season 2 was released in 2014.
An animated film titled A Certain Magical Index: The Movie – The Miracle of Endymion (とある魔術の禁書目録 エンデュミオンの奇蹟, Toaru Majutsu no Index: Endyumion no Kiseki) was released to Japanese theaters on February 23, 2013. It is based on an original story written by Kamachi and features the main characters from both Index and Railgun along with new ones designed by Haimura. Funimation has licensed the film in North America to be released in 2014. The film was released in North America on January 13, 2015.
The world of A Certain Magical Index is featured in the video game Dengeki Gakuen RPG: Cross of Venus for the Nintendo DS. Index appears as a supporting character, while Mikoto is a playable character which can be made accessible through playing the game and Accelerator is featured as a boss. A fighting game titled A Certain Magical Index, developed by Kadokawa Games and published by ASCII Media Works for the PlayStation Portable (PSP), was released on January 27, 2011.
Namco Bandai Games and Banpresto produced a crossover title, Toaru Majutsu to Kagaku no Ensemble (とある魔術と科学の群奏活劇, Toaru Majutsu to Kagaku no Ansanburu, A Certain Magical and Scientific Ensemble), for the PSP. The game, which takes place a few days before the events of the film, was released on February 21, 2013. Index also makes a cameo appearance in the Ore no Imōto ga Konna ni Kawaii Wake ga Nai PSP game, being another game based on a light novel series published by ASCII Media Works. A 2D fighting game developed by Sega titled Dengeki Bunko: Fighting Climax was released in March 2014 which brings together characters from light novel series published under Dengeki Bunko imprint. Mikoto Misaka is a playable character while Tōma Kamijō and Accelerator are assist characters.
Maiko Iuchi of I've Sound was in charge of the music in the A Certain Magical Index anime. Four pieces of theme music are used for the episodes: two opening themes and two ending themes. The first opening theme is "PSI-Missing" by Mami Kawada, and was used for the first 16 episodes during the television broadcast, but was changed to the first fourteen episodes in the DVD releases. The second is "Masterpiece" by Kawada, and is used in the subsequent episodes. The first ending theme is "Rimless (Fuchinashi no Sekai)" (Rimless 〜フチナシノセカイ〜, lit. "Rimless (Rimless World)") by Iku, and is used for the first 19 episodes. The second is "Chikaigoto (Sukoshi Dake Mō Ichido)" (誓い言 ～スコシだけもう一度～, lit. "Oath (Just a Bit, One More Time)") by Iku, and is used in the subsequent episodes. A single for "PSI-Missing" was released on October 29, 2008. A single for "Masterpiece" was released on February 4, 2009.
The first opening theme song of A Certain Magical Index II is titled "No Buts!" by Kawada and the single was released on November 3, 2010. The second opening theme is titled "See Visions" by Kawada and was released on February 16, 2011. The first ending theme song is titled "Magic∞World" by Maon Kurosaki and the single was released on November 24, 2010, and was used for the first 13 episodes of the season. The second ending theme is titled "Memories Last" (メモリーズ・ラスト, Memorīzu Rasuto) by Kurosaki and was released on March 2, 2011, and is used for the subsequent episodes of the season. Iuchi is once again in charge of music direction and composition for Index II. The main theme song of the film A Certain Magical Index The Movie: Endymion's Miracle is "Fixed Star" by Kawada and the single was released on February 20, 2013. The opening theme for the A Certain Magical Index PSP game is "Answer" by Kurosaki, the B-side song included on her single "Magic∞World".
A drama CD was released as a mail-in order of Dengeki hp volume 48 and was released later in November 2007. There are two parts to the story: the first part originally aired as a radio drama in Dengeki Taishō and starts with an encounter with the mysterious self-proclaimed "former" magician by Tōma Kamijō and Index in the family restaurant after Misaka decides to go back due to urgent business. The second part is about Mikoto Misaka and Kuroko Shirai with their "urgent business", and a duel request by a Level 3 Psychic girl from Tokiwadai.
Weiß-Schwarz has adapted Index and Railgun into a playable and collectible card game. An official visual book of A Certain Scientific Railgun was released on July 24, 2010, bundled with a DVD containing a five-minute bonus episode.
In Kadokawa Shoten's Light Novel Award contest held in 2007, A Certain Magical Index was a runner-up in the action category. The light novel series has ranked three times in Takarajimasha's light novel guide book Kono Light Novel ga Sugoi! published yearly: fourth in 2009, ninth in 2010, and first in 2011. Between the 2010 and 2014 issues of Kono Light Novel ga Sugoi!, Mikoto Misaka was voted the No. 1 ranked female light novel character, now five times in a row, a feat not achieved by any other light novel character. Tōma Kamijō was voted the No. 1 ranked male light novel character in the 2011 issue.
In May 2010, it was reported that A Certain Magical Index became Dengeki Bunko's No. 1 bestseller. Later that year, it became the fifth best selling light novel in Japan, beating other popular series such as Full Metal Panic!, Shakugan no Shana, and Haruhi Suzumiya. The Mainichi Shimbun reported in November 2010 that the novels have sold over 10 million copies.