The changing and increasing competitive environment brings new opportunities and risks to enterprises. Developments in the field of information and technology are among the main factors in the formation of this environment. It is an inevitable necessity for enterprises to continuously adapt to the developments in business environment, possibly through digital transformation, in order to seize opportunities and to reduce newly emergent risks. Despite all changes in the competitive environment, employees are still one of the most essential resources for enterprises today. As in the case of all business resources, it is extremely important to use the labor force effectively and efficiently and to keep employee performance at the highest level to achieve business goals and objectives in a sustainably competitive way. The internal service quality approach provides a practical analytical framework to evaluate employee performance. According to the approach, employees are considered as internal customers, the services provided to them by other employees as interior services, and the quality of these services as internal service quality. Improvement of internal service quality and ensuring digital transformation might have a positive effect on the employee performance. Hence, the purpose of this study is to examine the impact of internal service quality on employee performance and the mediating role of digital transformation in this effect. To test research hypotheses formed in the study, data were collected from 133 employees working in an import-export company by online survey method. The data were analyzed through frequency analysis, confirmatory factor analysis and regression analyses. Hayes Process plug-in of SPSS was used to test the mediation hypothesis. According to the research findings, internal service quality and digital transformation have a positive and significant effect on employee performance. Another finding of the study is that digital transformation has a partial mediating role in the effect of internal service quality on employee performance.