We investigated a gastroenteritis outbreak in Erzurum city, Turkey in December 2012 to identify its cause and mode of transmission. We defined a probable case as onset of diarrhoea (⩾3 episodes/day) or vomiting, plus fever or nausea or abdominal pain during 19-27 December, 2012 in an Erzurum city resident. In a case-control study we compared exposures of 95 randomly selected probable cases and 95 neighbourhood-matched controls. We conducted bacterial culture and real-time multiplex PCR for identification of pathogens. During the week before illness onset, 72% of cases and 15% of controls only drank water from antique neighbourhood fountains; conversely, 16% of cases and 65% of controls only drank bottled or tap water (adjusted odds ratio 20, 95% confidence interval 4·6-84, after controlling for age and sex using conditional logistic regression). Of eight stool specimens collected, two were positive for Shigella sonnei, one for astrovirus, one for astrovirus and norovirus, and one for astrovirus and rotavirus. Water samples from the fountains had elevated total coliform (38-300/100 ml) and Escherichia coli (22-198/100 ml) counts. In conclusion, drinking contaminated fountain water caused this multi-pathogen outbreak. Residents should stop drinking water from these fountains, and clean water from the water treatment plant should be connected to the fountains.