Recurring cases of financial distress among quoted deposit money banks in Nigeria have continued to raise concerns among regulators, investors, and other stakeholders despite several banking reforms and corporate governance policies introduced by the Central Bank of Nigeria. Weak ownership structures, insider abuses, excessive managerial control, and inadequate monitoring mechanisms have been identified as major causes of financial instability within the Nigerian banking sector. Consequently, this study examined the effect of ownership structure on financial distress of quoted deposit money banks in Nigeria. Specifically, the study investigated the effect of ownership concentration, managerial ownership, and institutional ownership on financial distress measured using the Altman Z-score. The study adopted an ex post facto research design. The population comprised thirteen (13) quoted deposit money banks listed on the Nigerian Exchange Group as at 2025, while eight (8) banks were purposively selected as the sample size. Secondary data were obtained from the annual reports and accounts of the sampled banks covering the period 2012–2025. Data were analysed using descriptive statistics and Random Effect Model regression analysis with the aid of STATA 15. The findings revealed that ownership concentration has a positive and significant effect on financial distress measured by Altman Z-score. Managerial ownership recorded a negative and significant effect, while institutional ownership exhibited a positive and significant effect. The study concluded that ownership structure significantly influences the financial stability of quoted deposit money banks in Nigeria. The study recommended balanced ownership structures and stronger corporate governance mechanisms within the Nigerian banking sector.