Purpose: This paper aims to investigate the extent of sustainability disclosure through websites at Spanish universities and analyse the determinants that affect such disclosure. Design/methodology/approach: This study uses as methodology a content analysis of the sustainability information disclosed by universities on their official websites in 2022 and a regression of ordinary least squares. Findings: Findings emphasise that Spanish universities have moderate levels of online sustainability disclosure, close to 50%, showing prevalent attention to dimensions concerning "organisation profile and governance", "economic aspects" and "labour practices", while "curriculum and teaching on sustainability topics" and "environmental" dimensions were less addressed. On the other hand, the findings indicate that public and larger universities are the ones most engaged in the online disclosure of information about sustainability issues. Likewise, universities led by female rectors exert a positive influence on sustainability disclosure on websites. Practical implications: The results could be useful for policymakers and regulators to implement and standardise sustainability reporting at higher education institutions, as well as for managers at universities who wish to increase the diffusion of sustainability outreach to satisfy stakeholders' demands and legitimise their actions in society. Originality/value: To the best of the authors' knowledge, this paper is the first Spanish approach to identify the explanatory factors for sustainability reporting in Spanish higher education institutions.