The region's medical officer, Dr. Valerie Jaeger, says the new patient is a woman under 30 who was not vaccinated.
Provincial health officials said earlier this week that one of the Niagara patients had travelled to Toronto for a youth gathering called "Acquire the Fire."
They asked those who attended the Feb. 6 and 7 event to check their immunization status.
Event organizers said about 1,000 people were present.
So far 17 cases of measles have been confirmed in Ontario, 11 of those in the Toronto area.
(CKTB, The Canadian Press)