Research Objective: The purpose of this study is to investigate the impact of bank market power on three key financial outcomes of companies, including financial performance, financing costs, and capital structure in companies listed on the Tehran Stock Exchange. This research is of an applied type and methodologically is within the framework of a descriptive-ex post-event approach.
Research Methodology: The statistical population includes small and medium-sized listed companies from 2014 to 2023, which were selected using a systematic elimination method and based on access to complete information.The data required for the research were extracted from companies' audited financial statements, Kodal reports, and official Central Bank information. After operationalizing the variables, they were analyzed using panel data regression and related diagnostic tests.Research findings: Research findings show that bank market power has a positive and significant effect on companies' operating performance (ROA), but this effect is not significant on market value-based performance (Tobin's Q), which indicates the operational role of banking relationships and the limitation of transferring this effect to the market valuation level.The results also shoFinally, the examination of capital structure showed that banks with market power are more likely to increase corporate leverage, thus reinforcing corporate dependence on debt.wed that bank market power significantly increases the financing costs of companies, suggesting that in centralized banking structures, companies face tighter credit conditions and higher interest rates.Conclusion: Given the importance of banking relationships in financing and investment decisions, the results of this study can be used by policymakers, banks, and corporate financial managers to improve operational efficiency, reduce financial costs, and design an optimal capital structure.