Koji Shinasaka is a Japanese manga artist and writer, born on January 6, 1970, in Tokyo, Japan. He is best known as the creator of the long-running mahjong-themed suspense series Tohai, which received a television anime adaptation in 2024. His career as a professional mangaka began after an editor discovered a self-published book he created following high school, leading to a debut four years later. Shinasaka has cited the manga Fist of the North Star as a major influence on his artistic style and the series Happo Yabure as the most influential work on his own mahjong manga.

Shinasaka's most significant and enduring work is Tohai, a seinen manga that follows a high school boy named Kei who competes in underground mahjong parlors. The original Tohai manga was serialized in Akita Shoten's Young Champion magazine from June 2006 to April 2011 and collected into twelve volumes. The story continued across three direct sequels: Tohai: Hitobashira-hen (2011-2017, 16 volumes), Tohai: Minagoroshi-hen (2017-2021, 10 volumes), and Tohai: Cold Girl (2021-present, 11 volumes as of December 2025). The series blends the strategy game of mahjong with organized crime, psychological tension, and high personal stakes. The fourth series, Cold Girl, remained ongoing as of late 2025.

Beyond the Tohai franchise, Shinasaka has created numerous other manga series. He is the author and artist of BW: Mahjong Hoshitori Densetsu (2013-2015, 3 volumes), a science-fiction mahjong story, as well as Haou Densetsu Lion (2010-2013, 4 volumes) and its sequel Haou Kessen Lion (2015-2017, 5 volumes), which focus on the character Doujima from the Tohai universe. He has also worked on horror and survival titles, notably Monkey Peak (2016-2019, 12 volumes), which he wrote, with art by Akihiro Kumeta, followed by the sequel Monkey Peak: The Rock (2019-2021, 9 volumes). Other works include the early series Mahjong Club (1993-1995, 2 volumes), Suzume no Uta (2012-2013, 1 volume), Akudō Warugaki (2017-2019, 5 volumes), and Rakugaki: Noroi no Yakata (2022-2024, 8 volumes). He has also contributed to anthologies for popular franchises, including Akagi and Touken Ranbu.

In an interview, Shinasaka has described Tohai as a story about using one's wits to fight against strong violence. He has expressed a love for science fiction and had originally planned a longer, more complex narrative for BW, involving characters fighting humanity and destroying the global financial system, though the series was cut short due to a lack of popularity. He has also noted the difficulty of creating believable cheating techniques in his stories, drawing inspiration from daily news and concepts like anti-counterfeit ink. The Tohai franchise was adapted into a live-action film in 2013. A 25-episode anime television series, Tohai: Ura Rate Mahjong Tohai Roku, produced by East Fish Studio, aired from October 2024 to April 2025. Outside of his professional work, Shinasaka enjoys playing mahjong and mountain climbing, with a goal to climb all 100 of Japan's Hyakumeizan mountains.