Koji Yanagi is a Japanese writer of mystery and thriller fiction, best known as the original creator of the Joker Game series. Born on September 20, 1967 in Mie Prefecture, Japan, Yanagi is a member of the Mystery Writers of Japan and has been active as a published author since 2001. His professional writing debut came with the novel Kogane no Hai in 2001, and in the same year he won the Asahi Award for New Writers for Gansaku "Botchan" Satsujin Jiken, a murder mystery parody of the classic Japanese novel Botchan.

Yanagi's most significant and internationally recognized work is the Joker Game series, also known as the D Agency series. The first volume, Joker Game, was published by Kadokawa Shoten in 2008 and became a critical and commercial success. The novel is a collection of interconnected short stories centered on a fictional spy training unit called D-Agency, established within the Imperial Japanese Army in the years leading up to World War II. For this work, Yanagi received the prestigious Mystery Writers of Japan Award for Best Novel or Linked Short Stories in 2009, as well as the Yoshikawa Eiji Prize for New Writers the same year. The series expanded to include three additional volumes: Double Joker (2009), Paradise Lost (2012), and Last Waltz (2015).

The success of the Joker Game novels led to a significant transmedia franchise, cementing Yanagi's role as an original creator in the anime and manga industry. The series has inspired multiple manga adaptations. The first, D no Maō, was illustrated by Kayoko Shimotsuki and serialized in a Shogakukan magazine from 2009 to 2010, later collected into three volumes. A second manga adaptation, simply titled Joker Game, was also illustrated by Kayoko Shimotsuki and ran in Big Comic Spirits from late 2014 to early 2015. A third manga, Joker Game the Animation, illustrated by Subaru Nitō, was published from 2016 to 2018. A live-action film adaptation, directed by Yu Irie, was released by Toho in January 2015. Most notably for anime audiences, a twelve-episode anime television series produced by Production I.G aired from April to June 2016, with Kazuya Nomura directing and Taku Kishimoto handling the screenplay. The anime adaptation brought Yanagi's characters and stories to a global audience, with an English dub produced by Funimation.

Yanagi's body of work extends well beyond the Joker Game franchise. He has written numerous standalone novels and short story collections, many of which blend mystery, historical fiction, and philosophical inquiry. Notable titles include Tokyo Prison, which was nominated for the Mystery Writers of Japan Award in 2007, and works such as Shinsekai (New World), King & Queen, Romance, and Knight and Shadow. His stories often incorporate real historical figures and events, demonstrating a consistent interest in the intersections of individual intellect, deception, and the rigid power structures of the early Showa era and other historical periods. The core philosophy of his most famous creation, the D-Agency spies who operate under the rule "don't die, don't kill," reflects his signature approach: portraying espionage as a cold, intellectual game of logic and invisibility rather than one of violent heroism. This focus on cerebral, character-driven historical thrillers has defined his artistic identity and made him a distinctive voice in Japanese genre fiction, with his work gaining further recognition internationally through its successful adaptations into anime and live-action cinema.