A motorcycle sidecar is a one wheeled vehicle that is attached to a motorcycle’s side. In this way, the motorcycle sidecar transforms the motorcycle from a two-wheeled vehicle to a three-wheeled vehicle.

The first motorcycle sidecar was meant to be a removable attachment that could be taken off whenever necessary. Today, the motorcycle sidecar is generally a permanent attachment. The wheel of the motorcycle sidecar does not line up directly with the rear wheel of the motorcycle. In addition, the wheel on the motorcycle sidecar is typically powered only by the motorcycle's rear wheel. This differentiates it from a motorcycle trike, in which both of the rear wheels share a common axle.

The frame of the motorcycle sidecar is rigidly attached to the frame of the motorcycle on one of its sides. The other side is supported by the wheel. The body of the motorcycle sidecar is usually large enough to fit one person, and also generally includes a trunk compartment in the back. Sometimes, the motorcycle sidecar also has a removable soft top. The motorcycle sidecar may also be designed solely for carrying cargo.

The motorcycle sidecar was particularly popular before the 1950s, during which time they provided users with a cheap alternative to cars. The motorcycle sidecar is also commonly used by police and in the army. German troops also used the motorcycle sidecar during World War II.

Today, there are only a few companies that still specialize in manufacturing the motorcycle sidecar. MZ in Germany, Harley-Davidson in the United States, Dnepr in the Ukraine, IMZ-Ural in Russia, Izhmash in Russian, Chang Jiang in China, and Watsonian/Squire in Gloucestershire still produce the motorcycle sidecar. Watsonian/Squire is the longest running manufacturer of the motorcycle sidecar, having been in business since 1912.

The motorcycle sidecar is also featured in many motorcycle races. Events in Trial, Motocross, and Streetracing each have motorcycle sidecar classes. These events are followed by fans in the United States, Europe, Australia, New Zealand, and Japan.