You will need: A valid U.S. passport. After June 16, 2019, U.S. citizens do not need a visa if they are traveling to Brazil for tourism, business, transit, artistic or sport activities, with no intention of establishing residence.
http://novayork.itamaraty.gov.br/en-us/visit_visa_(vivis).xml
No you do not need a Visa if you are an American Citizen... but you can walk in and double check in case things have changed
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