The tiger, named Nadia, is believed to be the first known case of an animal infected with Covid-19 in the US.
The Bronx Zoo, in New York City, says the test result was confirmed by the National Veterinary Services Laboratory in Iowa.
Nadia, along with six other big cats, is thought to have been infected by an asymptomatic zoo keeper.
The cats started showing symptoms, including a dry cough, late last month after exposure to the employee, who has not been identified.
“This is the first time that any of us know of anywhere in the world that a person infected the animal and the animal got sick,” Paul Calle, the chief veterinarian at the zoo, told Reuters news agency on Sunday.
There have been isolated instances of pets testing positive for the coronavirus elsewhere in the world, but experts have stressed there is no evidence they can become sick or spread the disease.
Mr Calle said he intends to share the findings with other zoos and institutions researching the transmission of Covid-19.
Read more at BBC