In late March 2014, the Regional Unit of the French Institute for Public Health Surveillance (InVS) (Cire) and the Aquitaine Regional Health Agency (ARS) were notified of an outbreak of acute gastroenteritis (AGE) and non-compliance with standards in untreated water samples in a municipality in the Pyrénées-Atlantiques department. Epidemiological and environmental investigations were conducted to identify the source of contamination and propose control measures. In the municipality, a cohort study was conducted at the middle school, along with a collection of data on school absences and microbiological analyses of stool samples from sick individuals. Investigations of the water supply system were also carried out. To describe the health impact of the outbreak, an analysis was conducted using data on drug reimbursements from the French National Health Insurance (AM). At the middle school, an attack rate of 93.1% was observed, with many cases reporting multiple episodes of acute gastroenteritis. The outbreak lasted from February to April, with a resurgence in June. Data on school absences and drug sales corroborated these findings. Several enteric viruses and parasites were detected in the patients’ stool samples. Contamination of the water supply network serving the middle school was identified and persisted for several months, despite continuous chlorination. In late June, backflow of water at the wastewater treatment plant was detected due to the absence of backflow preventers. These investigations confirmed the waterborne origin of the outbreak. This example highlights the need for municipalities to treat water and install backflow prevention devices in water systems.