One of the indicators that improves information asymmetry is the auditor's tenure. Therefore, in this study, the impact of the auditor's tenure on information asymmetry in companies listed on the Tehran Stock Exchange has been investigated. The main motive of this research was to reduce the information asymmetry due to the increase in the tenure of auditors in companies. The purpose of the research is to determine whether the duration of the auditor's presence in companies affects information asymmetry or not. The current research is practical in terms of its purpose and the research method is a posteriori event type. To test the research hypotheses, 101 companies have been selected from the Tehran Stock Exchange during the period of 2013-2014. In this research, in order to reduce the measurement error of the tenure of the auditor, the financial information of four periods before the time period of the research has also been used. By using multiple linear regression analysis, the results of the research indicate the impact of the auditor's tenure on information asymmetry. Therefore, the increase in the tenure of the auditor reduces the asymmetry of information compared to the information provided by the business unit, which confirms the auditor's expertise point of view and removes the concern about the auditor's independence point of view.