Dr. Thomas Farley, the city health commissioner, said the infectious mosquitoes were collected from Ferry Point Park in the Bronx, but so far, no human cases have been detected.
The health department has increased mosquito surveillance and mosquito larvae control in the affected area.
"West Nile Virus has returned to New York City, but simple precautions can help protect you and your family from the infection," Farley said in a statement.
He urged people to wear mosquito repellent when outdoors and to cover their legs if they go outside at dawn or dusk. Those over 50 should be especially cautious because they're more likely to suffer serious illness or death if they contract the virus.
West Nile virus can cause encephalitis, a serious inflammation of the brain or spinal cord, and can also cause a more moderate flulike illness with headache, fever and extreme fatigue.