Seventy cases - including 40 deaths - are confirmed, mostly in rural areas of the state of Minas Gerais. More than 300 cases are under investigation.
Vaccinations are being recommended for people travelling to Minas and other areas with confirmed cases.
What is yellow fever?
Yellow fever is a potentially fatal disease transmitted by mosquitoes.
Most of Brazil is considered at risk from yellow fever but the country has only seen a handful of cases in recent years.
- 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
The governor of Minas Gerais has declared a 180-day state of emergency.
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.
Around 5.5m vaccine doses have already been sent to five states that have confirmed cases or are at risk. The other 6 million ordered will arrive soon.
It is not clear what has caused the rise in cases.
Jimmy Whitworth, professor of international public health at the London School of Hygiene and Tropical Medicine, said: "It's unusual.
"The more cases you have, the more chance that it's going to light up and take off in urban areas."
The Brazilian authorities are taking the situation very seriously and appear to have enough vaccine stocks, he added.