The Tri-City Herald reports that the patient was a Walla Walla County man in his 20s.
The state Department of Health says he was likely exposed near his home and he was hospitalized.
Health officials say two other people from Washington - a man from King County in his 70s and a woman from Grays Harbor in her 50s - were infected while out of the state.
Reports of some additional infections are being investigated.
The virus, which is most commonly spread by mosquitoes, can cause fever, headache or body aches. A small percentage of patients become seriously ill.
The most active year for human West Nile virus infection in Washington was 2009, with 38 cases.