A soap-box cart is a wheeled vehicle frequently built by children (and some adults) for recreation. The name soap-box cart reflects the fact that wooden crates used to transport soap were, historically, very often the starting point for building these, though many other materials are used. Other names for the same thing are Billy cart (usually the term used in Australian English), Go-Cart, Trolley, soap-box racer, etc.

In addition to being something often built by children, there are organised competitions and races ("soapbox derby") that often engage the enthusiasm of adults. However, these things are usually entered into in a spirit of fun rather than serious competition. Often these will be fund-raisers for charity.

A typical soap-box cart is constructed of wood, and has four wheels, arranged as a fixed rear axle, and a steerable front beam axle - usually with a very simple single central pivot. A seat is arranged at the back, and perhaps the seat area is enclosed, as in the original soap-box design. More sophisticated designs might employ a fully-enclosed body. The types of wheels employed vary according to what can be obtained easily - wheels from baby carriages, pushchairs, prams, and discarded bicycles being common. Ready-made wheels are also available from hardware suppliers. Steering is typically effected using foot control, or perhaps using a rope connected to the ends of the steerable beam (which can then double as a useful manual pulling device). More sophisticated steering methods are rarely seen. Brakes are also not commonly used, though some soap-box cart racing contests require these. Often a simple friction brake operated by a lever which bears on one of the tyres is all that is needed, which will be of dubious effectiveness.

Soap-box carts are unpowered, and are either pushed by willing helpers, or are run down a suitable slope. Races will usually take place downhill and the most efficient and skillfully driven cart will win - gravity applying equally to all.

Soap-box carts make great construction projects for children, requiring only an intuitive sense of engineering, and a few basic construction skills. There are also predesigned kits available, though for many this defeats the purpose of the exercise.