The case "reminds us that we are moving into the high-risk season with the arrival of hot weather," state public health officer Dr. Mark Horton said in a statement released Thursday.
The unidentified man, from Kern County, was hospitalized. The state Department of Health Services declined to provide further information.
The federal Centers for Disease Control and Prevention has reported four other cases as of June 13: two in Texas and one each in Mississippi and Colorado.
Last year, the CDC counted about 3,000 West Nile cases nationwide, including 119 deaths.
The virus is transmitted to humans through mosquito bites. Most infected people never get sick, but up to about 20 percent develop mostly mild flu-like symptoms. Severe symptoms, sometimes including fatal brain inflammation, are rare.
Since 1999, when the virus first appeared in this country, 782 West Nile deaths have been reported nationwide.