Norovirus (NoV) is the leading cause of nonbacterial foodborne outbreaks of gastroenteritis. Individuals who are asymptomatically infected may act as reservoirs to facilitate transmission of NoV. This retrospective study was conducted to identify the viral agent and investigate potential transmission of NoV infection in a foreigner patient who had severe acute gastroenteritis after having a meal in a restaurant in Chiang Mai province, Thailand. The fecal specimens collected from the patient and 26 restaurant staffs were tested for the presence of gastroenteritis viruses by PCR and RT-PCR. The NoV positive cases were confirmed by real-time PCR and IC kits. The sensitivity of detection of IC kit B, as compared to those of real-time PCR, could detect the viral load down to at least 2.1 × 10 copies/g of stool. The diarrheic patient was infected solely with GII.3 NoV without co-infection with any other gastroenteritis viruses while 4 staffs (15.4%) were positive for different NoV strains (3 with GII.4 and 1 with GII.17) and all were asymptomatic. Interestingly, the GII.3 NoV strain detected in fecal sample of the patient was closely related to GII.3 NoV strains detected previously in fecal samples of children hospitalized with acute diarrhea in Chiang Mai, in the same year and the same geographical area where the patient was infected, suggesting the circulation and transmission of GII.3 NoV in this area. In conclusion, our data indicated that the patient was infected with GII.3 NoV and the virus was not directly transmitted to the patient by asymptomatic food handlers instead it might be transmitted by consumption of NoV-contaminated food provided by the restaurant. In addition, the existence of NoV in asymptomatic food handlers could be a potential source of NoV transmission. Therefore, strict adherence to hand hygiene practices should be reinforced to prevent foodborne outbreaks.