It has been well established that sustainable development is a critical issue worldwide. Countries should consider the conservative consumption of available resources while maintaining their desired levels of economic growth. This study aimed to develop a global sustainability index for the Group of Eight + Five (G8+5) countries by using the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS). The index is based on three criteria: the Gross Domestic Product, carbon dioxide emissions, and the World Happiness Index, which constitute the economic, environmental, and social dimensions, respectively, of sustainability. The results indicate that the TOPSIS has a competitive advantage over other multiple-criteria decision-making techniques. They also reveal the criteria weight sensitivity of the developed index. This suggests that policymakers and researchers should pay special attention to criteria selection and weighting.