The counties are Miami-Dade in south Florida, Hillsborough in the Tampa Bay region, Lee County in southwest Florida and Santa Rosa County in the Florida Panhandle.
Scott directed the state's surgeon general to declare the public health emergency, and for state officials to pay special attention to mosquito spraying in residential areas in those counties.
Although officials believe Zika virus is only spread by mosquitos, the first known case of transmission in the United States, which was reported in Texas on Tuesday, is suspected to have been contracted through sex and not a mosquito bite.
The virus, which is rapidly spreading in the Americas, has been linked to a spike in birth defects in thousands of babies in Brazil.
The World Health Organization declared a global health emergency on Feb. 1, citing a "strongly suspected" relationship between Zika infection in pregnancy and microcephaly, a condition marked by abnormally small head size that can cause permanent brain damage in newborns.
Scott noted in an executive order that the Aedes aegypti mosquito that transmits Zika is common in Florida, where a warm climate provides for a nearly year-round mosquito season. The virus was first identified in 1947 in Uganda.
Al Jazeera and Reuters