Rivaldo Barbosa, head of the Rio police homicide division, identified the victim as 38-year-old Victoria Tcaciuc and said she was killed Feb. 20. A hotel employee found her body the same day.
Police have arrested a suspect who is seen in security camera video entering the hotel in Rio's Copacabana neighborhood, Barbosa said.
It is unclear when Tcaciuc arrived in Brazil, but Barbosa said at a news conference that she was planning to visit at least four other cities before returning to the United States on March 4. She was divorced and had no children.
The press office of the U.S. Consulate in Rio said Tcaciuc was a contract employee of the State Department. Citing privacy act considerations, the consulate did not provide her hometown or other information.
William Ostick, a spokesman with the State Department in Washington, said in an email: "We were deeply saddened to learn of the death of Department of State contractor Victoria Tcaciuc. We express our deepest condolences to her friends."
The arrested man, identified as Alessandro Rufino Oliveira Carvalho, has acknowledged that he was with Tcaciuc in the room but denies killing her, Barbosa said.
Barbosa said Carvalho left the hotel less than an hour after entering and traveled to Sao Paulo. He was arrested Thursday when as he was returning to Rio.
The two apparently met at a crafts fair, had lunch and then went to the hotel.
"We suspect that he killed her because she refused to have sex with him," Barbosa said.
The O Globo newspaper quoted Carvalho as saying: "In the hotel all she wanted to do was smoke a cigarette and watch television. There was no sex and when I left she was very much alive."