Water supply companies are increasingly expected to contribute to achieving Sustainable Development Goals (SDGs), being among the leading actors in environmentally sensitive sectors. Accordingly, they are also called to increase their transparency on SDGs to gain legitimacy for their operations and acquire social consensus. To this end, social media may represent a valid instrument for implementing two-way communication with stakeholders, going beyond the boundaries of traditional financial or annual reports. With this in mind, this study extends academic knowledge on the SDG disclosure practices of water supply companies via social media. The ultimate paper’s goal is to examine the possible determinants of such disclosure practices. A manual content analysis was conducted on a sample of 64 Spanish water supply companies’ official Twitter accounts to determine the SDG disclosure levels. Under the lens of legitimacy theory, different regression models were implemented to test some firm characteristics’ impact on the SDG disclosure level provided by Spanish water supply companies via Twitter for the year 2021. Results reveal that Spanish water supply companies are still slow to realise the potential of social media to increase their engagement with stakeholders and legitimise their position in society concerning their commitment to achieving SDGs. More profitable companies, as well as non-state-owned ones, appear more prone to disclose SDG information. This paper adds to existing research by exploring SDG disclosure in water companies, an underdeveloped area. Unlike previous studies focusing on conventional channels, this analysis examines SDG disclosure on social media, a novel perspective yet to be extensively studied.