This study examined the effect of Environmental, Social, and Governance (ESG) disclosure on firm performance and competitive advantage among listed financial service firms in Nigeria. Adopting an ex-post facto research design, panel data were collected from 31 firms over the period 2020–2024 and analyzed using fixed and random effects regression models. ESG disclosure was measured using environmental, social, and governance scores, while firm performance and competitive advantage were proxied by Return on Assets (ROA) and finance cost respectively. The findings revealed that environmental and governance disclosures had no significant impact on firm performance or competitive advantage in listed financial service firms in Nigeria. However, social disclosure exhibited a significant negative relationship with finance cost, indicating its role in enhancing competitive advantage through reduced financing costs. The results further showed that firm-specific factors such as age and leverage significantly influenced performance outcomes. Overall, the study concludes that ESG disclosure does not uniformly affect firm performance in Nigeria, although social disclosure plays a critical role in improving firms’ financial positioning. The study recommends improved ESG reporting standards and deeper integration of sustainability practices into corporate strategy to enhance long-term value creation.