There are few studies focused on the voluntary disclosure of sustainability information in seaports, which are organizations with particularities that differ from traditional companies. The main objective of this study is to create indexes of the extent and quality of environmental sustainability disclosure for a sample of 54 ports belonging to the specific environmental initiative of the port sector called EcoPorts, for the period 2021. The data was collected using the content analysis technique and then treated quantitatively. The results suggest that the overall performance of these organizations, both in terms of the extent to which they disclose environmental information and the quality of that information, is considered weak. This study contributes theoretically to the development of literature focused on the disclosure of the environmental dimension of Corporate Social Responsibility in relation to one of the main sectors responsible for the international transportation of goods. It also helps to raise awareness of the subject, particularly among decision-makers and port authorities.