The Southeastern United States, also known as the American Southeast or simply the Southeast, is a geographical region of the United States located in the eastern portion of the Southern United States and the southern portion of the Eastern United States. The region includes a core of states that reaches north to Maryland and West Virginia, bordering the Ohio River and Mason–Dixon line, and stretches west to Arkansas and Louisiana.

There is no official U.S. government definition for the region, and it is defined variably among agencies and organizations.

The history of the present-day Southeastern United States dates to the dawn of civilization in approximately 11,000 BC or 13,000 BC. The earliest artifacts from the region were from the Clovis culture. Prior to the arrival of European colonialists, Native Americans occupied the region for several hundred years during the Woodland period.

The first Europeans to arrive in the region were conquistadors associated with the Spanish Empire. In 1541, Hernando de Soto journeyed through the southeast and crossed the Mississippi River. The region hosted the first permanent European settlement in North America, with the Kingdom of England establishing Jamestown, Virginia in 1607. Four southeast states, Georgia, North Carolina, South Carolina, and Virginia, were among the Thirteen Colonies who sent delegates to the Second Continental Congress in Philadelphia, which declared independence from the British Crown, which was established with the colonies' victory over the British in the American Revolutionary War.

During the American Civil War, the Confederate States of America consisted of the southeastern states of Florida, Alabama, Georgia, Mississippi, Tennessee, South Carolina, North Carolina, Virginia, Louisiana, and Arkansas. Texas was a Confederate State that isn't defined as part of the southeast region. Missouri and Kentucky had dual competing Unionist and Confederate state governments in the Confederate government of Missouri and Confederate government of Kentucky, while Maryland and Delaware were neutral southern border states that remained with the Union. West Virginia split from Virginia during 1863, and also served as a border state that remained with the Union while being contested by the Confederacy and Confederate government of West Virginia. Following the Reconstruction era in the 1870s, many Southeast state legislatures passed Jim Crow laws. The American segregation era spanned from the late 19th century to the mid-1960s.

In the mid-to-late 20th century, the Southeast saw many shifting changes take place population wise and economically. The Southeast started to see a spur of new economic growth in the 1930s, which stemmed from New Deal policies such as the Tennessee Valley Authority and the Fair Labor Standards Act instituting a minimum wage for the entire nation. World War II further helped growth within the Southeast, as military bases and military production drew workers from farming or low-wage industries into a new economy. With farming mechanization picking up to speed in the 1940s, promotion of different industries, and federal spending on defense and space programs, the Southeast saw further economic transformation in the ensuing years and decades of the 20th century.

The Southeast benefitted from its business and warm climate in its population growth during the late 20th century, as it helped in attracting job seekers and retirees from other U.S. regions. Florida in particular went from being the 27th most populated U.S. state in 1940 with 1.9 million residents, to being the 4th most populated U.S. state and having nearly 13 million residents in 2000. Southeast states such as North Carolina and Georgia saw large population growth increases as well during the late 20th century.

The United States Geological Survey defines Southeastern United States as including the states of Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Puerto Rico, the United States Virgin Islands, and Virginia.