We are among the first to study the association between board gender diversity and carbon assurance. Based on a sample of 3,373 unique U.S firms during the period 2007–2019, we document a positive and economically meaningful association between women on boards and carbon assurance. The documented effect is stronger for firms that have three or more women on the board of directors, supporting the critical mass hypothesis. We find consistent results when we employ propensity score matching estimates and difference-in-differences using sudden deaths of female directors as a potential shock to address endogeneity concerns.