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.

The company found mainstream success in a series of loosely historical action games, the flagship titles of which are the Dynasty Warriors and Samurai Warriors series, also known as the Musō series. Koei 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 merged with Tecmo on April 1, 2010 and combined both companies as one under the name Tecmo Koei Games (with Koei as the actual surviving corporation).