A three wheeler is a vehicle with three wheels, either "human or people-powered vehicles" (HPV or PPV or velomobiles) or motorized vehicles in the form of a moped, motorcycle, All terrain vehicle (ATV) or automobile. Other names for three-wheelers include Trikes, Tricars and Cyclecars. The term Tricycle is used somewhat interchangeably, but the term three-wheeler is more often applied to motor vehicles.

Many three-wheelers which exist in the form of motorcycle-based machines are often called trikes and often have the front single wheel and mechanics similar to that of a motorcycle and the rear axle similar to that of a car. Often such vehicles are owner–constructed using a portion of a rear–engine, rear–drive Volkswagen "Beetle" in combination with a motorcycle front end. Other trikes include ATVs that are specially constructed for off road use. Three-wheeled automobiles can have either one wheel at the back and two at the front, (for example: Morgan Motor Company) or one wheel at the front and two at the back (such as the Reliant Robin).

Three-wheeler cars, usually microcars, are often built for economy reasons, or as was the case in the UK, to take advantage of tax advantages, or as in the US to take advantage of the lower safety regulations, they are being classed as motorcycles. As a result of their light construction and often relaxed pollution reqirements, leading to higher efficiency, three-wheeled cars are usually very economical to run.