["Corporate Social Responsibility and Environmental Management, EarlyView. ", "\nABSTRACT\nThis paper examines how sustainability reporting practices in Nigeria's banking sector evolved following the Central Bank of Nigeria's (CBN's) introduction of the Nigerian Sustainable Banking Principles (NSBP) framework in 2012. Using qualitative content analysis, this study examines annual reports and standalone or integrated sustainability reports for all 12 banks listed on the Nigerian Exchange (NGX), across five observation years (2011, 2016, 2018, 2020, and 2022). Disclosures are coded against nine NSBP principles and interpreted using institutional theory, focusing on coercive, mimetic, and normative isomorphic pressures, while legitimacy theory provides complementary insights into early voluntary practices. Sustainability reporting in the pre‐policy period was largely voluntary, fragmented, and focused on CSR‐related activities. In the post‐policy period, disclosures increasingly reflected mandated NSBP reporting, consistent with observed coercive (regulatory) pressure and subsequent field‐level convergence. Average NSBP‐mandated disclosure increased from approximately 26% in 2011 to 64% in 2016 and to 93% by 2022. Sustainability reporting appears to have evolved through three distinct phases: a pre‐policy voluntary phase, an early post‐policy compliance phase driven primarily by regulatory mandates, and a later emerging institutionalization phase characterized by greater reporting convergence and broader adoption of NSBP‐aligned disclosure practices. The observed reporting patterns are consistent with an evolving interplay of coercive, mimetic, and normative institutional pressures, with coercive regulatory pressure serving as the primary driver of change. This study contributes longitudinal evidence from an emerging African banking context by demonstrating how sector‐specific sustainability regulation shaped the institutionalization and convergence of sustainability reporting over time. It further contributes by operationalizing the NSBP as a sector‐specific coding framework and by distinguishing increased reporting convergence from evidence of substantive sustainability performance. The findings suggest that sector‐specific reporting frameworks, implementation guidance, and industry‐level engagement can support reporting convergence and greater consistency in sustainability disclosure practices. The study also indicates that regulators may need to complement disclosure requirements with monitoring, assurance, and outcome‐based measures to assess whether reported sustainability practices are substantively implemented. The study demonstrates how sector‐wide sustainability reporting frameworks can strengthen transparency and accountability within emerging market banking systems, particularly in relation to environmental and social risk management, financial inclusion, women's economic empowerment, and broader sustainable development objectives.\n"]