Light Yagami (Japanese: 夜神 月ライト, Hepburn: Yagami Raito) is the main protagonist of the manga series Death Note, created by Tsugumi Ohba and Takeshi Obata. He is portrayed as a brilliant but bored genius who finds the Death Note, a supernatural notebook that allows the user to kill anyone by knowing their name and face, after it is dropped by the Shinigami Ryuk. With the power of the Death Note, Light eliminates those he deems morally unworthy of life, masterminding a worldwide massacre as the mass murderer Kira (キラ). He believes he is making the world a better place by purging it of all its impurities, taking the stance of the "God of the new world" and thus fulfilling his goal. Over the course of his efforts to create a world free of crime, wherein he would rule as a godlike figure, Yagami is pursued by law enforcement groups such as the NPA and a world-renowned detective named L.

In the anime adaptation, he is voiced by Mamoru Miyano in Japanese and by Brad Swaile in the English version. In the live-action film series, he is portrayed by Tatsuya Fujiwara with Swaile reprising his role as his English dub voice; he is portrayed by both Kenji Urai and Hayato Kakizawa in the musical; in the TV drama, he is portrayed by Masataka Kubota; his counterparts in the American film are portrayed by Nat Wolff and Margaret Qualley. Yagami's portrayal in the anime was met with praise from critics.

Tsugumi Ohba, the story writer of Death Note said that his editor suggested the family name "Yagami" for Light, which he did not feel "too concerned" about regarding its meaning (the Kanji for "Yagami" are "night" and "god"). Light's given name is written with the Japanese character for "moon," an example of what is coincidentally called a kira kira name. In the final scene of the manga, after Light's death, a group of Kira worshippers appear under the light of the moon; Ohba describes liking this scene as it created "deeper significance" for Light's name.

Takeshi Obata, the artist of Death Note, said that he had "no trouble" designing Light as the character description presented to him, "A brilliant honors student who's a little out there," was "clear and detailed". As the weekly serialization continued, Obata simplified the design by subconsciously removing "unnecessary" lines and felt that he became "better" at drawing Light. When Chapter 35 appeared and the editor informed Obata that Light loses his memories, Obata had to draw Light in a similar manner as he appeared in Chapter 1; Obata said "It was like I had to forget everything I had learned." Obata said that he used "a lot of effort" to design Light's wardrobe. According to Obata, he encountered difficulty imagining the clothing of "a brilliant person," so he looked through fashion magazines. Obata envisioned Light as a "smart and formal guy" who wears formal shirts. Most of Light's clothing in Death Note is "fitted", and Obata avoided jeans. For color illustrations, Obata assigned "a lack of color or clear" to Light, as he felt these color themes best fit his "atmosphere".

Light's character was slightly altered as he was adapted into the anime. Toshiki Inoue, the series organizer for the anime, confirmed in an interview that the anime production team intended to portray Light as "pitiful", as well as confirming an intent to portray his death more sympathetically than in the manga. As stated by Inoue and the anime producers, Light's death scene in the anime came to emphasize the pathos of "a sad person" who ruined his own life.

Shusuke Kaneko, director of the film, intended for Light to appear sympathetic at the beginning of the film; when Light first gains the Death Note, Kaneko "was careful" to have Light react in a manner "as you and I would". Kaneko changed the story involving Light gaining his first notebook as he felt that the audience "would have a hard time sympathizing" with Light if the scene remained the same as it was in the manga. Kaneko added that as he portrayed Light as "being enthralled" as he "becomes more cruel" to make the audience members feel that they could "do the terrible things he does" even if the members do not sympathize with Light.

Tatsuya Fujiwara said that he felt difficulty portraying Light Yagami in the film series because of the lack of "action" and because Light has no signature mannerisms and therefore has his feelings displayed by his face; Fujiwara added that he struggled conveying Light's "incredible intelligence" and that the performance would appear "very empty or simplistic" if Light received an improper portrayal. Fujiwara explained that he wanted Light to cry in a particular scene even though Kaneko told Fujiwara "Light doesn't cry" since Fujiwara believed that the scene would feel "more honest"; Kaneko used the take.

Kaneko designed Light's room to reflect the character's personality by making it clean and neat and filling it with legal, criminal history, foreign, and academic books. The original version of Light's room included a stereo; Kaneko replaced it with a vacuum cleaner to reflect Light's "clean-freak self".