In recent years, the issue of environmental sustainability has garnered significant attention, prompting many companies to use social media as a platform to communicate their sustainability initiatives to external stakeholders. Nonetheless, research on the impact of corporate environmental sustainability communications via social media remains limited. Grounded in social identity theory and utilizing largescale observational data from X (formerly Twitter), we find a counterintuitive result: corporate sustainability communications via social media are associated with an average 29.11% decrease in daily engagement volume across companies’ regular communications. Interestingly, however, we find that response messages (as opposed to broadcast messages) can offset this adverse effect. Our findings indicate that managers should be cautious when implementing sustainability communication strategies on social media, as these communications may produce unintended negative consequences. This study extends the corporate social responsibility communication literature and provides strong empirical evidence for adverse consumer reactions to sustainability communications.