An underbone is a type of motorcycle that uses structural tube framing with an overlay of plastic or non-structural body panels and contrasts with monocoque or unibody designs where pressed steel serves both as the vehicle's structure and bodywork. Outside Asia, the term underbone is commonly misunderstood to refer to any lightweight motorcycle that uses the construction type, known colloquially as step-throughs, mopeds^{[citation needed]} or scooters.

An underbone motorcycle may share its fuel tank position and tube framing, along with fitted bodywork and splash guards with a scooter while the wheel dimensions, engine layouts, and power transmission are similar with conventional motorcycles.

Unlike conventional motorcycles, underbones are mostly popular in Asia and Greece. In Indonesia, the fourth most populous country in the world, and the largest country in Southeast Asia, almost half the population have a motorcycle, most of which are underbones and scooters. (120 million in 2018, compared to 16 million cars).

The appearance and frame of an underbone are quite different from that of a standard motorcycle, but the powertrain is functionally almost identical. The underbone engine is positioned between the rider's feet but the rear wheel is driven by a regular motorcycle secondary chain drive. Styling considerations only mean that the chain-driven nature of the machine tends to be concealed under sheet-metal covers to a greater extent than that of motorcycles.

In only one significant respect does the underbone differ from the conventional motorcycle - there is no frame member or fuel tank between the seat base and the headstock. Generally, the fuel tank is located under the seat. The underbone arrangement improves the ease of mounting and dismounting and contributes to the cross-gender appeal of the layout as an around-town shopping and commuter vehicle. The engine displacement of a typical underbone motorcycle is generally between 50 cc and up to about 150 cc, though much larger ones are available. Currently, production underbone model with the largest engine displacement is the SYM VF3i, which is powered by a fuel-injected 183 cc SOHC 4-valve water-cooled engine.

Historically, underbones popularized semi-automatic transmissions and indicator systems. In other respects, the technical sophistication of underbones tended to lag those of larger motorcycles, but nowadays they are increasingly equipped with similar electronic ignition, fuel injection, etc.

Underbones do not have large footrests on the sides (compared to scooters/mopeds), this makes it easier, safer and more comfortable to ride on rough roads or off-road. As the rider can more easily stand up or put both feet on the ground while riding, to shift centre of gravity, smooth out rough roads with knees in half-standing, maintain stability and control. Similar to techniques commonly used in dirt biking or mountain biking.

Underbones may have spoked or alloy wheels that are spindle mounted and usually sized to fit small conventional motorcycle frames. These provide much better road-holding and braking than scooters, though it does make punctures more difficult to repair