This study develops a synthetic sustainability indicator (SSI) to assess corporate sustainability performance using data from Spanish IBEX 35 companies. Utilizing sparse principal components analysis (sPCA), the study condenses 65 Environmental, Social, and Governance (ESG) variables into a single indicator, addressing the lack of standardization in CSR evaluations and enabling comparisons across companies and sectors. The results show that environmental practices are the most significant factor (37.30%), followed by labor practices (19.23%) and corporate governance (18.52%). The study’s key contribution lies in offering a quantifiable, transparent method for evaluating corporate sustainability. The SSI provides valuable insights for investors and analysts, promoting data-driven decision-making and encouraging corporate improvements in sustainability performance.