Renewable energy is a global interest area in achieving sustainable development. Renewable energy sustainability has been assessed using the most commonly used dimensions of this concept: economic, environmental, social, and institutional dimensions. In this paper, we designed a composite index named the Renewable Energy Sustainability Index. The proposed index may represent a national monitoring mechanism that points out the strengths and weaknesses of a state in terms of renewable energy. The data were normalized by calculating the z-score. We tested the proposed index on a selection of 15 European countries ranked by final energy consumption and with different levels of development. The Kayser-Mayer-Olkin values were above the 0.700 limit, which indicates the robustness of each dimension. The proposed index reveals the development stages of renewable energy sustainability and can provide solutions to increase the sustainability of a country by improving positive impact indicators and mitigating negative impact indicators.