The centre of Hurricane Irma has hit mainland Florida, just south of Naples, amid warnings of storm surges as high as 15ft (4.5m).
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".
The storm hit Marco Island at 15:35 local time (19:35 GMT).
The National Hurricane Center tweeted that people in the area of Naples and Marco Island should move away from the coast as storm surges of up to 15ft were possible.
The storm is now heading for the Tampa Bay area, with a population of about three million. It has not been hit by a major hurricane since 1921.
"We are about to get punched in the face by this storm," Tampa Mayor Bob Buckhorn said.
Irma had earlier barrelled through the Florida Keys, a chain of low-lying islands to the south.
The whole of the southern tip of Florida has seen high winds, driving rain and storm surges,
About 2ft (60cm) of water has been seen in Miami's financial district, where one major street resembled a river, but the worst damage is expected on the west coast.
Two cranes have collapsed in high winds in the city.
Which other areas have already been hit?
Irma is the most powerful Atlantic storm in a decade, and has already caused widespread destruction on several Caribbean islands:
Cuba: Officials have talked of "significant damage", without giving further details, but no casualties have been reported. Electricity is out across the capital, Havana
St Martin and St Barthelemy: Six out of 10 homes on St Martin, an island shared between France and the Netherlands, are now uninhabitable, French officials say. They said nine people had died and seven were missing in the French territories, while four are known to have died in Dutch Sint Maarten
Turks and Caicos Islands: Widespread damage, although extent unclear
Barbuda: The small island is said to be "barely habitable", with 95% of the buildings damaged. Antigua and Barbuda Prime Minister Gaston Browne estimates reconstruction will cost $100m (£80m). One death has been confirmed
Anguilla: Extensive damage with one person confirmed dead
Puerto Rico: More than 6,000 residents of the US territory are in shelters and many more without power. At least three people have died
British Virgin Islands: Widespread damage reported, and five dead
US Virgin Islands: Damage to infrastructure was said to be widespread, with four deaths confirmed
Haiti and the Dominican Republic: Both battered by the storm, but neither had as much damage as initially feared