Countries around the world have launched public disclosure programs to stimulate citizen participation in environmental governance, yet little is known about when such participation affects regulation and pollution. We conducted a national-scale field experiment that randomly varied how citizen appeals about violations of pollution standards were sent to regulators or the violating firms. We find that, appealing a firm’s violations to the regulator publicly through social media increased both regulatory oversight and firm compliance, which reduced subsequent violations by 60% and air and water pollution emissions by 12.4% and 3.8%, respectively. In contrast, appealing to the regulator through private channels only caused a marginal improvement in environmental outcomes. Additionally, we randomly varied the proportion of firms subject to appeals at the prefecture-level and find that pollution appeals filed against the treatment firms did not crowd out local governments’ regulation of control firms. Analysis of ambient pollution data and back-of-the-envelope calculations both suggest that encouraging more public participation in environmental governance would lead to significant improvements in China’s aggregate environmental quality.