The woman, 24, was accompanied by U.S. embassy officials to a hospital in the state capital of Cuiaba, a statement from the Mato Grosso state civil police said on Saturday, the third day of the month-long tournament.
The woman and her boyfriend were staying in the home of someone they met during Friday's World Cup celebrations and she woke up screaming with a man on top of her, the statement said.
Several people were interviewed about the incident, including the owner of the home, but no one has been arrested, police said.
A U.S. embassy spokeswoman in Brasilia said the embassy was aware of reports of the case but could not comment due to privacy laws.
Cuiaba, in central Brazil, usually sees few tourists, but will host three more games: on June 17, between Russia and South Korea; on June 21, between Nigeria and Bosnia; and on June 24, between Japan and Colombia.
Brazil has taken pains to ramp up security during the World Cup, with about 100,000 police patrolling the 12 host cities complemented by nearly 60,000 soldiers.
The tournament has gone more smoothly than many expected but there have been reports of robberies in several host cities, including 12 cases involving tourists registered by police in Cuiaba on Friday.
The rape and kidnapping of a foreign student in host city Rio de Janeiro in April also shocked residents and raised concerns about World Cup security.