Outbreaks of acute gastroenteritis (AGE) caused by the consumption of contaminated tap water are identified every year in France, but the actual number is likely underestimated. Nevertheless, the public health impact can be significant when a community is affected (20 to 50% of consumers falling ill, based on past outbreaks). To improve detection, Santé publique France is implementing a nationwide surveillance system described in this protocol. This system complements water quality monitoring (sanitary water testing and monitoring of water suppliers). It helps improve understanding of the health impact of these events (surveillance objective) and target measures to secure and control water distribution systems against infectious risks (prevention objective). The developed method enables the automated detection of clusters of medicalized GEA cases (GEAm) located within the same water distribution unit, based primarily on data from the National Health Data System (SNDS) sourced from the Health Insurance system (prescription details) and the SISE-Eaux d’alimentation database. The identified clusters trigger targeted environmental investigations of the water networks supplying the cases to confirm the waterborne origin and identify the circumstances of contamination. A classification of clusters to assess the level of plausibility of a waterborne origin is proposed following the environmental investigations (strong, probable, possible, undetermined). To facilitate the management of the surveillance system and to implement the aforementioned method operationally, a tool for reviewing alerts and reporting regional investigation results (EpiGEH) was rolled out by Santé publique France to all regional teams in late 2018. Implementing the surveillance system requires the involvement of a range of partners, each with their own area of expertise: Santé publique France for detecting outbreaks and evaluating prevention measures; health authorities (ARS) for conducting environmental investigations and recommending corrective and preventive measures; and drinking water system operators for ensuring the safety of water production and distribution systems.