Nowadays the increasing demand for sustainable mobility has fostered the introduction of innovative propulsion systems also in the public transport sector in order to achieve a significant reduction of pollutant emissions in highly congested urban areas. Within this context this paper describes the development of the HYBUS, an environmental friendly hybrid bus for on-road urban transportation, which was jointly carried out by Pininfarina and Politecnico di Torino in the framework of the AMPERE project.The first prototype of the bus was built by integrating an innovative hybrid propulsion system featuring a plug-in series architecture into the chassis of an old IVECO 490 TURBOCITY. The bus is 12 meters long and has a capacity of up to 116 passengers in the original layout. The project relied on a modular approach where the powertrain could be easily customized for size and power depending on the specific application. Furthermore this flexibility could pave the way to a significant reduction of investment costs since it could allow the revamping of obsolete vehicles in the fleet of public transportation companies, extending their end of life.After a preliminary description of the main features of the hybrid architecture, the paper assesses, through numerical simulations, the fuel economy potential of the vehicle in real world driving conditions. The promising results of this study led to the development of the abovementioned prototype which was extensively tested in the city of Genoa, Italy, an urban context which is extremely demanding because of its routes featuring a continuously variable altitude and significant grades. The experimental data collected during the test campaign confirmed the simulations forecasts, highlighting significant fuel economy benefits.