PurposeGreenwashing is defined as the overstatement of companies’ environmental disclosures relative to their performance. This paper aims to develop a greenwashing measure, examines its relationship with environmental performance and investigates the mitigating effects of Japanese firm-level corporate governance characteristics (corporate structure, board leadership, foreign share ownership, ratio of independent directors and ratio of directors’ variable compensation) and third-party assurance of environmental information on the extent of greenwashing.Design/methodology/approachThis paper analyzes a sample of 420 firm-year observations from the period between 2018 and 2019 from Japanese listed companies that responded to the CDP Climate Change survey via probit/logit and multivariate panel data regression models.FindingsThis paper finds that the probability of engaging in greenwashing is negatively associated with environmental performance, which supports the reliability of the study’s greenwashing measure. Japanese firm-level corporate governance characteristics are ineffective at mitigating greenwashing. This paper also finds that assurance carries a significant risk of being exploited by companies involved in greenwashing to increase the degree of their overstatement.Practical implicationsThe findings have significant implications for investors, who should increase scrutiny and skepticism of environmental disclosures, particularly from companies with poor environmental track records. Japanese companies should consider strengthening their corporate governance to ensure the effective oversight of environmental disclosure and performance. Regulators and standard setters should implement stricter guidelines for and oversight of environmental information assurance.Originality/valueNo empirical study has examined the effectiveness of Japanese corporate governance characteristics and environmental disclosure assurance on the mitigation of greenwashing.