Monitoring potentially devastating coral-eating crown-of-thorns starfish (COTS) populations at scales relevant to management is a challenging task. Here, we investigated a citizen science approach to detect COTS outbreaks and prioritize management responses. Between 2014 and 2018, 38 000 COTS were recorded through 641 online observation reports submitted across New Caledonia, Vanuatu and Fiji by private stakeholders (51%), NGOs (22%), business operator (11%), research/government agencies (16%). COTS were observed in multiple areas, including in remote/inhabited reefs where they had never been reported. A three-level classification was developed to discriminate risk areas and propose operational guidelines to streamline management actions. About two-thirds of reports had low abundances (<10 starfish sighted) and could be addressed with low priority. Verification surveys at 65 reef sites confirmed outbreaks in half of the cases, along with high peak densities (7 000 ind.ha(-1)). Combining professional and non-professional observers increased the detection range (+27%) and the number of COTS detections (+129%). Citizen reports were eventually followed by removal campaigns organized within diverse institutional frameworks. While citizen monitoring has intrinsic limitations, we advocate that it constitutes a complementary and promising approach to support the ongoing management efforts in all countries affected by COTS.