Introduction: Following reports of several cases of acute gastroenteritis among participants at a company dinner on January 9, 2015, various investigations were conducted to characterize the outbreak, identify the pathogen, and determine the source of contamination. Method: A retrospective cohort study was conducted on January 16, 2015, among the 655 participants using an online questionnaire. A case was defined as any participant in the meal who experienced vomiting or diarrhea between the meal and January 16, 2015. The association between the onset of symptoms and food consumption was assessed using Poisson regressions. Stool and oyster samples—the main suspected food source, whose origin was determined through a traceability investigation—were collected for microbiological analysis. Results: Among the 216 respondents (33%), 99 cases (attack rate: 46%) were identified. The median incubation period was 38 hours. Among the cases, 68% experienced diarrhea and 57% experienced vomiting. Oyster consumption was associated with gastrointestinal symptoms, showing a dose-response relationship: 1–3 oysters: incidence ratio (IR) = 10.3 (95% CI: [4.7–22.4]); 4–6 oysters: IR = 10.9 [5.0–24.0]; 7 or more oysters: IR = 12.0 [5.3–27.0]. A norovirus was detected in stool and oyster samples. Discussion and conclusion: These investigations quickly suggested a link between gastrointestinal symptoms and oyster consumption, leading to the recall of the suspected oysters from the market.