This study examined the effect of sustainability reporting on the financial performance of listed oil and gas firms in Nigeria, focusing on Oil Spill Remediation Disclosure (OSRD), Decommissioning and Asset Retirement Disclosure (DARD), and the moderating role of the Petroleum Industry Act Compliance Index (PIACI). Anchored on Legitimacy Theory, the study adopted an ex post facto research design using panel data from the annual and sustainability reports of listed Nigerian oil and gas firms. Tobin's Q served as the measure of financial performance, while panel regression analysis was employed to examine the hypothesized relationships. The findings indicate that OSRD and DARD have positive and significant effects on Tobin's Q, suggesting that transparent petroleum-specific sustainability disclosures enhance firm value by improving investor confidence and reducing information asymmetry. PIACI also exerts a positive and significant direct effect on financial performance, indicating that regulatory compliance strengthens corporate legitimacy and stakeholder trust. However, PIACI does not significantly moderate the relationship between sustainability reporting and financial performance. The study concludes that petroleum-specific sustainability reporting is a strategic driver of market value rather than merely a compliance obligation. It recommends strengthening environmental disclosure practices and aligning sustainability reporting with the Petroleum Industry Act and ISSB Standards to enhance transparency, investor confidence, and long-term firm value.