Urban sustainability focuses on the more efficient management of the resources in cities and their regions. It is a challenge to develop a framework that it easy to replicate and can be used by any city in the world. In this paper, we aimed to develop a methodological approach for assessing urban sustainability of Romanian cities. We designed an index (Urban Sustainable Development Index) based on aggregating data from 15 indicators structured on the 3 sustainability domains, with equal weights per each domain. General results present medium and low USDI scores which indicate that cities are almost unsustainable. Very few cities have high USDI scores, making them almost sustainable cities due to the environmental or social contribution. Cities used as case studies have problems related mainly to green areas, waste management, crimes, health care infrastructure and low GDP. The assessment framework used for determining urban sustainability helped us identify some problematic domains. The proposed methodological approach can be replicated by cities worldwide, being recommended especially for cities with difficult access to data. The analysis uses representative information for urban sustainability that can be easily replicated and used for monitoring by stakeholder and decision makers at local level.