Star Marta scored twice from the penalty spot and Cristiane also had two goals as the Brazilians were ahead, 4-0, after only 56 minutes.
Daniela Alves' goal in the 76th minute closed out the scoring.
"Competing in this Pan Am Games has been a privilege for our players," said U.S. head coach Jillian Ellis. "This is a young team and one cannot beat the experience of playing Brazil in Maracana Stadium in front of a packed house. Brazil had an exceptional level and style of play that contributed to their great success. We will learn from all of it."
July 27 in Rio de Janeiro
Brazil 5 USA 0. Goals: Marta pen. 19, pen. 56, Cristiane 30, 48, Daniela Alves 76.
USA -- Naeher, Taylor, Marshall, Wilmoth (Enyeart, 46), Fountain; Wright, Heath, Edwards, Nogueira, O'Hara, Cheney.
Brazil -- Andreia, Aline, Renata Costa, Elaine, Rosana (Katia, 83), Daniela Alves, Miraildes, Marta, Cristiane (Delma, 78), Andreia Santos, Tania.
Referee: Fernando Cabrera (Uruguay).