Washington state public health officials said the patient was isolating and not at risk of spreading the virus
Officials in King County, which covers Seattle, are working to identify who the man has been in contact with while he was infectious, and are contacting people they described as being “potential low risk exposures”.
The man did not require hospitalisation and is isolating at home, they said.
Washington is the fifth US state to report a confirmed or suspected case of the Monkeypox virus.
Testing carried out at the Washington State Public Health Laboratory showed the man had tested positive to the family of viruses known as orthopoxviral.
Further testing will be carried out by the Centers for Disease Control and Prevention.
There has been one confirmed case of Monkeypox in the United States in a Massachusetts man.
Suspected cases have also been reported in Utah, Florida and New York.
Seattle & King County public health official Jeff Duchin said in a statement there was no evidence that Monkeypox was spreading locally, but didn’t rule out the possibility.
“People should understand that the disease can affect anyone and those who are most at risk are those who have had close physical contact with someone with monkeypox. The risk is not limited to men who have sex with men,” Dr Duchin said.
World Health Organisation (WHO) official David Heymann said on Monday the leading theory on how the current outbreak is being spread was sexual contact among gay and bisexual men who attended two recent raves in Spain and Belgium.
“We know monkeypox can spread when there is close contact with the lesions of someone who is infected, and it looks like sexual contact has now amplified that transmission,” Dr Heymann said.
President Joe Biden delivered a more reassuring message to reporters in Tokyo on Monday, a day after he had said “everybody should be concerned” about the disease.
He said the smallpox vaccine was effective against Monkeypox, and the US has sufficient doses on hand to deal with an outbreak.