Ian has regained strength over the Atlantic Ocean and become a hurricane again, the National Hurricane Centre (NHC) said.
The storm had been downgraded to a tropical storm as it weakened over Florida.
Hurricane Ian is now forecast to hit South Carolina on Friday as a Category 1 hurricane.
The storm is about 240 miles (390 kilometres) south of Charleston, South Carolina. A hurricane warning has been issued for the entire South Carolina and the southern end of the North Carolina coast.
A tropical storm warning is in effect for other parts of the North Carolina coast and the Georgia coast.
The hurricane had wind speeds up to 75 miles per hour (120 kilometres per hour) on Thursday afternoon. By Friday, winds are forecast to reach up to around 80 mph (129 kph).
Storm surge in parts of South Carolina could reach up to seven feet (2.1 metres) on Friday as some spots could see up to 12 inches (30 centimetres) of rain, NHC said.
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