Koei Co., Ltd. was a Japanese video game publisher, developer, and distributor founded in 1978. The company is known for its historical simulation games based on the novel Romance of the Three Kingdoms, as well as simulation games based on pseudo-historical events.

Koei found mainstream success in its series of loosely historical action games such as Dynasty Warriors and Samurai Warriors, also known as the Musō series. It also owned a division known as Ruby Party, which focuses on otome games.

On April 1, 2009, Koei merged with Tecmo to create the Tecmo Koei Holdings holding company. After operating as subsidiaries of Tecmo Koei Holdings for exactly a year, Koei and Tecmo were combined as a single company on April 1, 2010 to form Tecmo Koei Games (with Koei as the actual surviving corporation).

Koei was established in July 1978 by Yōichi Erikawa [ja] (also known as Kou Shibusawa [ja]) and Keiko Erikawa. Yoichi was a student at Keio University, who decided to pursue his interest in programming after his family's rural dyestuffs business failed. The company has since remained located in the Hiyoshi area of Yokohama.

Koei initially focused on personal computer sales and made-to-order business software. In 1982, the company released the erotic title (eroge) Seduction of the Condominium Wife (団地妻の誘惑, Danchi Tsuma no Yūwaku), which was an early role-playing adventure game with color graphics, owing to the eight-color palette of the PC-8001 computer. It became a hit, helping Koei become a major software company. In March of the same year, Koei released Underground Exploration, the earliest known Japanese RPG. In 1983, it released Nobunaga's Ambition (信長の野望, Nobunaga no Yabō), a historical strategy game set during the Sengoku period of Japanese history. The game went on to receive numerous awards. The company continued producing more games set against the backdrop of world history, including Romance of the Three Kingdoms, set during the Three Kingdoms period of Chinese history, and Uncharted Waters (大航海時代, Dai Kōkai Jidai; lit. Great Navigation Era), set in Portugal during the Age of Exploration.

In 1988, Koei established a North American subsidiary, Koei America Corporation, in California. This subsidiary localized Koei games for export to all territories outside Japan, and even produced original games and concepts with the leadership of designer Stieg Hedlund. These games included Liberty or Death, Celtic Tales: Balor of the Evil Eye, Gemfire, and Saiyuki: Journey West. After Hedlund's departure, this subsidiary ceased game development in 1995, focusing instead on localization, sales, and marketing.

Though none of Koei's historical simulations achieved mass market success, they acquired a loyal cult following that allowed them to remain profitable. They could reliably predict how many copies of their games would sell. This was especially important during the cartridge era; a surplus of unsold cartridges of a single game was often enough to bankrupt a company.

A Canadian subsidiary, Koei Canada, Inc., was established in early 2001, and a European subsidiary, Koei Limited, was established in early 2003 in Hertfordshire, United Kingdom. Koei also maintained subsidiaries in mainland China, Korea, Taiwan, and Lithuania. Koei later created a Singapore branch for game development, such as Sangokushi Online.