The National Hurricane Centre says Hurricane Michael has made landfall near Mexico Beach, Florida.
About 77,000 households are now without power.
Michael is an extremely dangerous category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale with maximum sustained winds of near 250 kilometres per hour or 155 miles per hour - just shy of category 5 at 157 mph.
Michael should weaken as it crosses the southeastern United States.
The hurricane is forecast to become a post-tropical cyclone on Friday, and strengthening is forecast as the system moves over the western Atlantic.