The extendable underrun protection device consists of a pair of transverse bars joined together by a pair of mobile scissor-shaped arms. The extendable underrun protection device is designed to be installed on the rear side of trucks. The device is approved in accordance with the provisions of Regulation 58 R - 03. The presence of the mobile arms, actuated by a pneumatic cylinders, allows the rear device to be folded inside the vehicle frame, facilitating the operations/manoeuvres that do not relate to road circulation. During road circulation (therefore, with the device extended), the underrun protection device is designed to prevent vehicles (e.g. vehicles and motorcycles) may be jammed under the truck in a collision. This protection device is built with a heavy-duty structure.