The 2016 Summer Olympics are being held in Rio de Janeiro from August 5, 2016 until August 21, 2016. The Paralympic games are being help between September 7th and September 18th.
Brazil is waiving all visa requirements for US, Australian, Canadian and Japanese citizens visiting Brazil under a temporary visa waiver program that allows for 90 days of visa-free entry between the dates of June 1st until September 18th.
Citizens of many countries do not need a visa to visit Brazil, but all US citizens are normally required to get one; they normally cost $160 to get one and typically take as long as 4+ weeks to obtain (unless you use our Expedited Brazil Visa service that is)
Henrique Eduardo Alves , the Brazilian Tourism Minister said the hope is that the temporary visa waiver program, which runs from 6/1 thru 9/18, will increase leisure travel from the four countries (US, Canada, Australia, Japan) to Brazil by 20 percent over the duration of the visa waiver program.
Summary: Any US citizen arriving in Brazil between 6/1/16 and 9/18/16 can stay 90 days without a visa.