The paper explains the modern role of internal audit in credit institutions, and special emphasis is placed on its role in the context of sustainability reporting. In the last few years, the role of internal audit in certain companies, as well as credit institutions, has been expanding because of the introduction of new regulations related to non-financial reporting, i.e. sustainability reporting, which requires institutions to report on ESG factors. Credit institutions that are obliged to publish sustainability reports can use the internal audit function to ensure compliance with the new regulation and to ensure the publication of accurate and complete information. The aim of this work is therefore to determine the importance of the internal audit function in these institutions, especially in the context of sustainability reporting, and to shape the guidelines for its future action in this regard. After analyzing research on the involvement of internal audit in reporting on the sustainability of business entities, it was concluded that internal audit helps with its engagements in compiling sustainability reports, but despite this, most internal auditors have not yet included sustainability reports in their audit plans. Due to the new regulation, which stipulates the obligation of external audit of sustainability reports, this is expected to change. In the last part of the work, guidelines for internal auditors for auditing sustainability reports by stages of the audit process were formulated.