Four people are believed to have directly contracted Zika virus from mosquitos while in the United States - the first evidence of local transmission on the mainland.
Mosquitos likely infected three men and a woman within a square mile of Miami, according to Governor Rick Scott.
While the type of flies are yet to be identified, health officials have ruled out other means of transmission, including travel to another country with a Zika outbreak and sexual contact.
"We have worked hard to stay ahead of the spread of Zika and prepare for the worst," Mr Scott said.
"We will continue to put every resource available to fighting the spread of Zika in our state".
Despite the outbreak, pregnant women are not being advised to move out of the suspected transmission area.
Zika's greatest risk is believed to be posed by infection in pregnant women, given its ability to cause microcephaly in babies, a condition defined by small head size that can lead to developmental problems.