A water-fuelled car is a hypothetical motor car that uses ordinary water as its fuel.

This is not the same concept as a steam engine. A steam engine uses water to transmit energy from the fire or other heat source to the pistons or turbine that do the work of turning the engine.

This is not the same concept as a hydrogen car, though it often incorporates some of the same elements. To fuel a hydrogen car from water, energy from a power plant is used to generate hydrogen from electrolysis, which is then either burned in the car's engine or merged with oxygen to create water via a fuel cell. Both methods produce energy used to create motion. The car ultimately receives its energy from the power plant, with the hydrogen acting as an energy carrier. They should also not be mistaken for hydrogen fuel injection which introduces small quantities of hydrogen into a 'lean' fuel-air mix that would not normally burn. Hydrogen fuel injection systems are currently sold for both gasoline and diesel engines.

The water-fuelled car, on the other hand, would have to create or extract energy from the water itself with no other energy input.

Water fuelled cars have been mentioned in history books, newspaper and popular science magazines, and urban legends since the 1800s. Some accounts are of an engine that runs on water but the idea is suppressed by either Big Oil or automakers in order to safeguard their profits. "The Water Engine", a David Mamet play later made into a film, tells such a story. Claims for water fuelled power sources are often used to fraudulently extract money from gullible investors.