This study investigates the influence of ownership structure on banks’ cost efficiency in Poland’s banking sector, emphasising the comparison between foreign, private, and state ownership. We use data from 2005 to 2021 to explore whether the ownership type impacts banks’ cost-to-income ratios. Our findings reveal that government and domestic-owned banks have higher cost efficiency than their private and foreign counterparts. The study also identifies other determinants that positively affect banks’ efficiency, including banks’ profitability, loan loss provisions, and solvency ratio, or that affect them negatively, such as implementing the Basel requirements and the banks’ age. These insights are critical for regulatory authorities, policymakers, and banking sector managers, highlighting the impact of ownership structure on bank cost-efficiency and the broader implications for the Polish banking industry’s competitive landscape and regulatory framework.