Touhou Gensou Mahjong (loosely translated to "Touhou Illusion Mahjong", also known as Touhou Unreal Mahjong) is an indie riichi mahjong game developed and published by D.N.A. Softwares for the PC on March 8, 2009.
A fangame of Team Shanghai Alice's Touhou Project series, Touhou Gensou Mahjong is a four-player riichi mahjong game with characters and music from the series. The game is notable for each character having a unique special ability, such as changing score values or replacing hand tiles, that is charged by discarding certain tiles. Characters can be unlocked through a single-player story mode.
The game received numerous updates throughout the years, adding additional modes (such as three-player, free-play, and online multiplayer), new characters, and new themes. This includes Touhou Gensou Mahjong: 3rd Generation on August 13, 2011, Touhou Gensou Mahjong 4 on May 26, 2013, and Touhou Gensou Mahjong 4N on May 11, 2014.
It was later ported to the Nintendo Switch, published digitally by Mediascape (as part of the "Play, Doujin!" brand) in Japan on January 30, 2020. This version is notable for having ZUN himself as a playable guest character (who was added as paid DLC on December 24, 2020).