Honkaku Yo-nin Uchi Pro Mahjong: Mahjong Ou (loosely translated to "Authentic 4-Player Hit Mahjong: Mahjong King") is a riichi mahjong game developed by Warashi (along with Mahjong Kobo) and released by Taito for arcades (using their PS1-based Taito G-NET hardware) in Japan in 1999.
One of the few arcade games at the time to support the traditional four-player version of riichi mahjong, Mahjong Ou features the likenesses of 12 professional mahjong players and a game A.I. that is advertised as both strong and fast. It features a story mode where players play against various mahjong professionals in order to get their autographs.
The game later received a port to the Sony PlayStation on May 25, 2000, published by Warashi. This version includes a revamped interface. It later received a PSP sequel in 2006, known as Honkaku Yo-nin Uchi Pro Mahjong: Mahjong Ou Portable.
The game also received a handheld adaptation for the Game Boy Color on February 19, 1999 under the name Honkaku Yo-nin Uchi Mahjong: Mahjong Oh. Developed and published by Warashi, this version later received a spin-off known as Mahjong Jo-ou ("Mahjong Queen").