This paper develops a new sustainability index for evaluating the sustainability performance of mining companies. The index indicates a mining company’s overall performance level, relative to other companies in terms of its sustainability performance. To facilitate the use of the index as a sustainability performance management tool, three most important criteria for measuring the sustainability-related performance of mining companies are identified. With the use of linguistic terms, subjective assessments of qualitative performance measures are represented with fuzzy numbers. Based on the concept of optimality, a fuzzy multicriteria decision making approach is developed to obtain a sustainability performance index for each company. This sustainability performance index helps mining companies to understand their strengths and weaknesses in terms of sustainability performance criteria, and identify relevant areas for sustainability improvement.