Using survey data of 107 internal auditors, we provide empirical evidence about internal auditor’s role in environmental, social, and governance (ESG) disclosure and assurance. So far, academics and practitioners have not agreed on the responsibilities of internal auditing in ESG matters. Our regression results give evidence for organizations’ and internal audit functions’ (IAFs) ESG maturity being associated with ESG reporting, while IAFs’ involvement in ESG shows a significant association with assurance of ESG reports. Our results support the notion that the environmental pillar is perceived as the core element of sustainability bearing the highest risk, therefore propelling the need for assurance. The imbalance between assurance and consulting activities on part of IAFs stresses the potential to position the IAF as a value-adding entity in a complementary relation towards external auditors. We further elaborate on IAFs’ non-involvement in ESG and demonstrate the respective lack of awareness on part of stakeholders. Our research provides valuable initial insights for internal audit practitioners in the field of ESG and implications for future research.