In this article we discuss the geopolitics over China's Mekong dams to provide an understanding of the challenging downstream impacts of its hydropower dams. When it comes to downstream countries struggling to secure their national water security while sustaining economic and diplomatic ties with Beijing, Beijing's actions reflect a huge mismatch between words and deeds. The relationship between Mekong riparian countries and their giant upstream neighbor is a complicated push-and-pull dynamic in which local voices and biodiversity conservation are often excluded. The growing citizen science–led collective grassroots initiatives in the Lower Mekong provide an opportunity for the downstream governments to broaden their response strategies. By analyzing the impacts of public participation in local environmental decision making and policy arenas, the article provides insights into the role of citizen science in networking science and grassroots movements to oppose ill-conceived projects in constructive and peaceful manners. It concludes with a discussion on how citizen science–led grassroots environmental movements can be translated into participatory diplomacy.