NisiOisin, stylized in all capital letters to emphasize its nature as a palindrome, is the pseudonym of a Japanese novelist, manga author, and screenplay writer born in 1981. After attending Ritsumeikan University, he left without graduating to pursue a career in storytelling, having initially wanted to be a manga artist before deciding to focus on writing. He made his debut in 2002 at the age of twenty with the novel The Beheading Cycle, the first entry in his Zaregoto series, which earned him the 23rd Mephisto Prize.

NisiOisin is best known as the creator of the Monogatari series. What began as a trilogy of short stories in 2005 expanded into a long-running novel series published in numerous volumes. This series was adapted into the highly successful anime Bakemonogatari, produced by studio Shaft, and its many sequels. The prequel story, Kizumonogatari, was adapted into a trilogy of anime films: Kizumonogatari Part 1: Tekketsu, Kizumonogatari Part 2: Nekketsu, and Kizumonogatari Part 3: Reiketsu. A compiled theatrical edition, Kizumonogatari: Koyomi Vamp, was also released. Beyond the Monogatari series, his other original works adapted into anime include the novel series Katanagatari, the manga series Medaka Box, the novel Jūni Taisen, the Zaregoto series, and the Pretty Boy Detective Club series.

His artistic identity is characterized by lengthy and witty dialogue filled with wordplay and manzai-style comedy. He focuses more on dialogue than physical appearance to define his characters, and he has a distinctive style of giving his characters bizarre and unusual names. His works often blend genres, incorporating elements of mystery, supernatural fiction, and comedy, and are filled with frequent references to other manga and anime. He cites a deep passion for JoJo's Bizarre Adventure and has stated that his writing was shaped by the works of Kiyoshi Kasai, Hiroshi Mori, Natsuhiko Kyogoku, Ryūsui Seiryōin, and Kouhei Kadono.

In the industry, NisiOisin is noted for his prolific writing speed and for blurring the lines between general literature and light novels. From 2009 to 2016, he was consistently ranked among the top ten best-selling authors in Japan, achieving the number one spot in both 2012 and 2014. As of November 2022, his novels and manga had over thirty-six million copies in circulation.