The Rio de Janeiro Health Department said in a statement that it aimed to vaccinate the state's population by the end of the year "as a preventative measure."
No cases of yellow fever have yet been recorded in Rio - Brazil's third most populous state - but the disease has taken hold in neighboring Minas Gerais, Espirito Santo and Sao Paulo.
"Rio de Janeiro is not an island. It is surrounded by states that have registered cases of yellow fever," Regional Health Secretary Luiz Antonio de Souza told the "O Globo" newspaper. "We are extending vaccinations throughout (the state) so that we can keep yellow fever out."
The regional government said it had already ordered 3 million of the 12 million doses that will be required.
Brazil is experiencing its largest yellow fever outbreak in almost 20 years. So far 113 people across the country have died of the mosquito-borne disease during the summer rainy season. At least 352 cases have been confirmed, with another 915 under investigation.
Yellow fever is carried by monkeys and can be transmitted to humans by mosquito bites. The disease causes shivers, aches, vomiting and jaundice, but in severe cases can result in kidney and liver failure and hemorrhages.
nm/jm (AP, AFP)