Hurricane Nate has made landfall near the mouth of the Mississippi River in Louisiana, the US National Hurricane Center (NHC) says.
The storm, with maximum sustained winds of 85mph (137km/h), is moving north, and a second landfall is expected on the Mississippi coast later.
Louisiana, Mississippi, Alabama and parts of Florida earlier issued warnings and evacuation orders.
Nate killed at least 25 people in Nicaragua, Costa Rica and Honduras.
The tropical storm has since strengthened and is now a category one hurricane on the Saffir-Simpson scale.
Although not as strong as last month's Maria and Irma, Nate is expected to bring strong winds and storm surges.
US President Donald Trump earlier issued an emergency declaration for Louisiana, allowing the state to seek federal help with preparation and possible relief efforts.
In Alabama, Republican Governor Kay Ivey has urged residents in areas facing heavy winds and storm surges to take precautions.
Five ports along the Gulf Coast have also been closed to shipping as a precaution.
Most oil and gas platforms in the US Gulf of Mexico have evacuated their staff and stopped production ahead of the storm.