You will need:
A valid U.S. passport
A Brazilian visa obtained before traveling to Brazil. Visit the Embassy of Brazil website for current visa information.
An immigration form filled out upon arrival. You must retain this form to hand back to immigration officials when you depart the country.
The U.S. government cannot assist you if you arrive in Brazil without proper documentation.
Special Entry/Exit Requirements for Minors: The Brazilian Embassy or Consulate may require a birth certificate and notarized travel authorization from both parents to issue a visa to a minor.
When departing Brazil, Brazilian minors (including U.S.-Brazilian dual citizens) who are not accompanied by both parents are required by law to prove that both parents authorized the travel. While non-Brazilian minors are not subject to the same requirement, immigration officials always have the right to stop individuals for questioning. There have been cases where non-Brazilian minors have been delayed or prevented from traveling when accompanied by only one parent or a third party. Please review the Brazilian government’s current visa information.
HIV/AIDS Restrictions: The U.S. Department of State is unaware of any HIV/AIDS entry restrictions for visitors to or foreign residents of Brazil.
Information from travel.state.gov on Jan 22, 2018