The Queen Elizabeth Hospital in Charlottetown says a number of patients in Unit 3 are experiencing symptoms including nausea, vomiting and diarrhea.
Amanda Hamel, a spokeswoman for Health PEI, says 12 patients were ill as of Wednesday.
She says tests are being conducted to determine the nature of the illness, and results are expected Thursday.
The hospital says the unit will be closed to all visitors until further notice and staff will be contacting the families of patients with more information.
Anyone coming to the hospital is being asked to wash their hands before and after entering the facility.