The man is a resident of Snohomish County, Wash., who experienced symptoms after returning from a trip to the region around Wuhan, China, where the outbreak began. He was hospitalized with pneumonia last week, and infection with the coronavirus was confirmed on Monday afternoon.
Local officials declined to identify the patient, who was said to be quite ill.
News of the first case in the United States comes amid growing evidence that the virus spreads from person to person, although it is not clear how easily.
The outbreak, which began in a seafood and poultry market in Wuhan, a city of 11 million, is spreading: Patients have been identified in Beijing, Shanghai and Shenzhen, as well as Taiwan, Japan, Thailand and South Korea.