The emergency declaration applies to Kern, Colusa and San Joaquin counties and will provide up to $1.35 million to help combat the spread of the virus, Schwarzenegger said.
West Nile is transmitted to humans through mosquito bites and so far this year has infected three times more people than it did in the same period in 2006, he said.
The disease's epicenter thus far is Kern County, which has logged two-thirds of the state's 56 West Nile cases this year, including an 85-year-old Shafter man and a 96-year-old Bakersfield woman who died last month.
Health officials announced the state's third and fourth fatalities this week: two elderly residents of San Joaquin and Colusa counties.
Schwarzenegger met with Kern County mosquito-control officials Thursday.
"Last year it was down, this year it has increased again," Schwarzenegger said. "The important thing is that we all go all out and we work together, the counties and the state, in order to get the job done and get rid of the virus."
Most infected people never get sick, but up to about 20 percent develop mostly mild flulike symptoms. Severe symptoms, including fatal brain inflammation, are rare.
Health officials recommend people avoid infection by staying inside at dawn and dusk, when mosquitoes are most active; wearing clothes that keep mosquitoes away from the skin; draining areas where mosquitoes can breed; and using insecticide with the chemical DEET.
In Sacramento County, authorities said Monday that West Nile had reached an epidemic rate and had to be combated with a mass aerial-spraying campaign -- often considered a last resort.
More than 55,000 acres of urban neighborhoods north of the American River were scheduled to be sprayed.
Health officials in San Jose said Thursday that a Santa Clara County resident had become infected -- their first case this year.
In Kern County, the new funds won't be enough to educate the public in time for the disease's high season in August and September, said Sen. Dean Florez, D-Shafter.
The state needs to provide a more consistent budget for eradication efforts in winter months, hire more vector control officers and coordinate surveillance efforts with real estate agents, who can provide updated information about vacant properties where standing water could provide the insects with a fertile breeding ground, he said.
"Next season could be quite possibly worse than this year unless we get a new infusion of money," Florez said.
Schwarzenegger said he was directing state agencies to take proactive measures and that more funds could be made available if needed.