They said there had been 63 confirmed cases of the mosquito-borne illness so far this year, up from seven in the whole of 2016.
Most of the cases have been in rural areas of Minas Gerais state, a Ministry of Health statement said.
The government has sent two million doses of yellow fever vaccines to the state.
The governor of Minas Gerais has declared a 180-day state of emergency.
What is yellow fever?
§ Caused by a virus that is transmitted to humans by mosquitoes
§ Difficult to diagnose and often confused with other diseases or fevers
§ Most people recover after the first phase of infection that usually involves fever, muscle and back pain, headache, shivers, loss of appetite, and nausea or vomiting
§ About 15% of people face a second, more serious phase involving high fever, jaundice, bleeding and deteriorating kidney function
§ Half of those who enter the "toxic" phase usually die within 10 to 14 days
Of the 63 confirmed cases in Brazil, 35 have proved fatal, Brazilian Health Ministry figures show.
That is the highest number of deaths since at least 2008, the year to which Ministry of Health records date back.
There have also been three confirmed cases in Sao Paulo, Brazil's most populous state, and one each in Espiritu Santo and Bahia, which both neighbour Minas.
It is not clear what has caused the rise in cases.