The fungus was found in Jefferson County, Georgia. USDA did not say what type of plant the soyarust was found on.
Rust will likely spread to new counties this growing season, but will not cause any economic damage to the crop because much of it already has been harvested, USDA said.
The fungus also has been discovered this year in Alabama, Mississippi, Florida and South Carolina, all states that grow a relatively small amount of soyabeans.
Soyabean rust was first found in the continental United States last November after spores were blown into the US Gulf Coast by a hurricane.
Rust can cut soyabean crop yields by up to 80 percent, if not detected and treated early.
The spores thrive in warm, humid weather under cloudy skies, and are spread by the wind.