The present study hypothesis resulted in the identification of a viable system of indicators that might play an important role in the measurement of sustainable development. The main purpose of our research is the development of a watershed sustainability index and its application for the Motru River basin. This catchment area covers an area of 1,895 km² within southwestern Romania. Applying the watershed sustainability index was based on the equal weighting scheme in order to aggregate the parameters (water availability per capita, variation of BOD, variation in the ratio of the agricultural area and the urban population, life quality index, and education index). Considering that basin management is a dynamic and holistic process, the state-pressure-response model was applied to the four components in a matrix scheme. As a result, for the Motru River basin, the obtained values of the watershed sustainability index were: 0.36 for five years and 0.51 for 10 years. The indicators with the lowest scores were hydrology and policy-education (0.17), while the environment indicator scored the highest (0.75). Applied to different time intervals, the watershed sustainability index emphasizes the evolution of watershed sustainability in order to help stakeholders and water managers in the decision-making process.