Ministry spokesman David Jensen says there are four confirmed cases of E. coli in the province, adding the patients became ill in late December.
Jensen says lab tests have confirmed the cases are linked to others in the Maritimes, adding the strain of E. coli is somewhat rare.
Jensen says officials don't know what's caused the outbreak and are continuing to investigate.
He says Ontario is working closely with officials in New Brunswick and Nova Scotia, as well as the Public Health Agency of Canada.
Officials in Nova Scotia have reported 10 cases of E. coli, while six cases have been documented in New Brunswick.
The strain of E. coli reported in those provinces is the same one found in the Walkerton, Ont., water disaster in 2000 that killed seven people.
Symptoms of the gastro-intestinal illness include severe cramps, bloody diarrhea and vomiting.