Transportation has significant economic, social and environmental impacts and is an important dimension of urban sustainability. Public transportation is recognisably a smart, efficient and sustainable mode of transportation. This paper describes a multi-criteria analysis to assess the sustainable performance of a bus transportation system in a Portuguese mid-sized municipality. The assessment is based on a list of 18 environmental, social, economic and institutional indicators. A group of specialists and political stakeholders defined the relative importance of these indicators. Results showed significant disparities in the sustainable performance of the analysed bus system. Political stakeholders were more sensitive to institutional indicators while specialists were more concerned with operational issues. The research also indicates that the institutional dimension is important for assessing the sustainability of a bus transportation system. The method proves to be useful for identifying the main problems of a bus transportation system and for supporting policies to mitigate these problems.