In June 2015, an outbreak of acute gastroenteritis (AGE) was reported among participants in an obstacle course near Nice in the Alpes-Maritimes department. A two-phase investigation was conducted to assess the origin and extent of the outbreak and to guide management measures. During the first phase, a Facebook post asked participants to report their illness via a dedicated messaging service provided by the Regional Health Agency (ARS). In phase 2, an online questionnaire was distributed for a retrospective study aimed at descriptive analysis and the identification of risk factors. Human and environmental microbiological analyses were conducted to identify the causative agent. An analysis of reimbursements for anti-diarrheal medications was conducted using health insurance data to support the epidemiological investigation. In Phase 1, out of 8,229 participants, at least 1,001 adults reported an acute gastroenteritis (AG) that resolved within 48 hours. In Phase 2, the identified risk factors for AG were: being younger, leaving after the first hour of the race, and ingesting mud. Twenty stool cultures were performed, all of which were negative. Only 4 samples were sent to the National Reference Center for Enteric Viruses. All tested positive for a norovirus of genogroup 1 and genotype 2 (GI.2), of strictly human origin. Drinking water analyses were within standards; the muddy water from the obstacles showed only bacterial contamination. The outbreak was caused by human-to-human transmission of a norovirus spread through muddy water contaminated by the feces or vomit of one or more infected individuals. A comprehensive risk assessment related to these races appears necessary to guide, on the one hand, health authorities in their vigilance and monitoring, and on the other hand, organizers in selecting the quality of materials used and in raising awareness of potential risks.