Ninja is an Arrow Dynamics/Vekoma steel roller coaster located in the Studio Backlot section of Six Flags St. Louis in Eureka, Missouri. It was originally built for and located at Expo 86 in Vancouver, British Columbia, operating as Scream Machine from May to October 1986. It began operating as Ninja at Six Flags St. Louis in 1989. The coaster was started by American manufacturer Arrow Dynamics, but when Arrow fell into bankruptcy, it was sold to Dutch manufacturer Vekoma, who finished it.
The ride features a loop, a sidewinder and a double corkscrew. The Ninja, like most modern roller coasters, requires a minimum of two employees to dispatch the train. One operates the main panel, which controls the restraints, gates and has a section for the mechanics. The other operator stands at the remote enable. Both operators have to do an all clear sweep and thumbs up, then press their buttons at the same time.