This paper tests the relationship between gender diversity on boards and firms’ environmental policy. Based on prior research, we predict that gender diversity on boards of directors should have a positive relationship with firms’ environmental policy. Moreover, firm character in terms of pollution creation likelihood moderates the relationship between gender diversity on boards and firms’ environmental policy. Analyzing data from 865 publicly listed firms in the United States, we found direct and significant empirical evidence for our predictions. According to the findings, we highlight the importance of gender diversity for the development of good firm environmental policy as well as for the improvement of corporate governance. Moreover, the more likely firms in a given industry are to cause environmental pollution, the more salient will be the beneficial effect of gender diversity on boards on firms’ environmental policy in the industry. Copyright © 2016 John Wiley & Sons, Ltd and ERP Environment