The global economic landscape is increasingly shaped by environmental impact as businesses worldwide are face with heightened pressure to comply with environmental standards, thereby making environmental conservation costs essential for maintaining profitability and building stakeholder trust. This study examined the effect of environmental conservation costs on financial performance of listed oil and gas firms in Nigeria. Using a longitudinal research design, data from the seven oil and gas firms listed on the Nigerian Exchange Group as at 31 st December, 2023, were collected from secondary sources, via the annual reports of the selected firms for a period of 2014 to 2023. Data collected were analysed using descriptive statistics and robust regression analysis. The results of the analysis showed that waste management cost as proxy of environmental conservation cost has significant positive effect on financial performance. Environmental remediation costs also showed a significant effect on financial performance, although the effect was negative. However, community development costs showed a positive effect, but it was found to be insignificant. Based on these findings, it is concluded that increased transparency in environmental cost disclosures can enhance financial outcomes, but some environmental liabilities may reduce profitability. The study recommends improving transparency in environmental reporting, strategic communication of environmental risk management, and maintaining strong community relations to enhance stakeholder trust and financial performance.