Underseal (often called undercoating ) is a thick resilient coating applied to the underbody or chassis of an automobile to protect against impact damage from small stones, which would rapidly chip ordinary paint, allowing rusting to begin.

Historically a bitumen-based compound was used, but after approximately a decade this becomes brittle and water can become trapped between the underseal and body metal, counterproductively creating a more favourable environment for rust than if no underseal was applied at all. wax-based underseals do not have this disadvantage but can be eroded and eventually washed off by enthusiastic use of a hot pressure washer. Rubber-based underseals are also sometimes used.

Vehicles for sale in some territories are not undersealed, because the climate is not sufficiently aggressive to warrant it. If such a vehicle is subsequently imported into a country with high rainfall, cold winters and where salting of the roads is common, application of underseal is strongly recommended to ensure the vehicle has a long life.