is a series of video games by Tecmo featuring the ninja Ryu Hayabusa as its protagonist. The series was originally known as in Japan. The word "gaiden" in the North American Ninja Gaiden title means "side-story" in Japanese, even though the Ninja Gaiden series is not a spinoff of a previous series. The original arcade version, first two NES games and Game Boy game were released as Shadow Warriors in PAL regions.