This paper examines whether the quality of external assurance affects corporate anti-corruption disclosure when firms are exposed to corruption risk. Corruption poses significant risk to society and concerns have been raised as to how firms may manage such risk and how to enhance transparency through anti-corruption disclosure. It is argued that firms facing higher corruption risks tend to provide less anti-corruption disclosure due to potential threat to firms’ reputation. However, it is unclear whether external assurance plays a role in fostering such disclosure. Using a sample of European Economic Area listed companies during 2018 and 2021 that are subject to the EU Non-financial Reporting Directive (NFRD), we find a negative association between firms’ corruption risk and the level of anti-corruption disclosure. We observe that over time, firms with greater corruption risk exposure actively reduce the level of disclosure and disclose with more repetitive content. We also find that assurance quality rather than mere existence of assurance is likely to play a role in mitigating the opaque anti-corruption disclosure practices among firms that face high corruption risk. However, the results only hold when external assurance is a firm’s voluntary choice rather than required. Our paper contributes to better understanding of whether focusing on the quality of external assurance may improve anti-corruption disclosure by firms with high corruption risk, and provides implications for regulatory and standard setters in light of the recent developments in reporting and assurance of non-financial information in Europe.