Building on agency and resource-based view theories, this study investigates the level of environmental disclosure (ED) practices of family versus non-family firms and explores the moderating role of board gender diversity. We test our hypotheses on a 3-year (2018–2020) panel data sample comprising 324 observations of Italian small- and medium-sized enterprises traded on the Euronext Growth Milan. Findings show that, compared to non-family firms, companies with a family firm status are characterized by lower levels of ED. Gender diversity on the board, however, moderates this relationship, reducing this gap, to the extent that the family firm status is associated with higher ED when the number of women directors is high enough to constitute a critical mass. We consequently contribute to the studies on family business, corporate governance, and corporate social responsibility.