Irma has been downgraded to a category three storm, but made landfall on Marco Island off Florida's west coast with winds of up to 120mph (192km/h).
More than 2.5m homes in the state are without power, and parts of the city of Miami are under water.
Irma has devastated parts of the Caribbean, killing at least 27 people.
Some 6.3 million people in Florida had been told to evacuate.
President Donald Trump described the hurricane as a "big monster". He praised the federal agencies involved with the storm and said he would go to Florida "very soon".
