This study aims to empirically investigate the association between residual audit fees and firm operational efficiency, using a sample of non-financial firms listed on the KOSPI and KOSDAQ markets with December fiscal year-ends from 2003 to 2022. The findings indicate a significant negative relationship between residual audit fees and operational efficiency. This suggests that higher residual audit fees are associated with lower operational efficiency, reflecting auditors' private evaluations of auditee accounting system quality. Robustness checks and additional analyses, including analyses of firms with high operating uncertainty, consistently support this finding. Furthermore, the results remain consistent even when controlling for traditional accounting quality variables. This study contributes to the literature by validating residual audit fees as a proxy for overall accounting quality within the Korean context and by bridging financial and managerial accounting perspectives.