The Mid-Atlantic states, also called middle Atlantic states or simply the mid Atlantic, form a region of the United States generally located between New England and the South. Its exact definition differs upon source, but the region often includes Delaware, Maryland, New Jersey, Pennsylvania, Washington D.C., and sometimes New York, Virginia and West Virginia.

The Mid-Atlantic has played an important role in the development of American culture, commerce, trade, and industry, yet it is one of the least self-conscious of American regions. It has been called "the typically American" region by Frederick Jackson Turner. Religious pluralism and ethnic diversity have been important elements of Mid-Atlantic society from its settlement by Dutch, Swedes, English Catholics, and Quakers through to the period of English rule, and beyond. After the American Revolution, the Mid-Atlantic region hosted each of the historic capitals of the United States, including the current federal capital, Washington D.C.

In the early part of the nineteenth century, New York and Pennsylvania overtook Virginia as the most populous states and the New England states as the country's most important trading and industrial centers. Large numbers of German, Irish, Italian, Jewish, Polish, and other immigrants transformed the region, especially coastal cities such as New York City, Newark, Philadelphia, and Baltimore, but also interior cities such as Pittsburgh and Buffalo.

New York City, with its skyscrapers, subways, and headquarters of the United Nations, emerged in the twentieth century as an icon of modernity and American economic and cultural power. It would suffer the brunt of the September 11 attacks, along with two other places in the Mid-Atlantic, Arlington, Virginia and Shanksville, Pennsylvania. By the twenty-first century, the coastal areas of the Mid-Atlantic were thoroughly urbanized. The Northeast Corridor and Interstate 95 linked an almost contiguous sprawl of suburbs and large and small cities, forming the Mid-Atlantic portion of the northeast megalopolis, one of the world's most important concentrations of finance, media, communications, education, medicine, and technology. Waves of Chinese, Indian, Mexican, Russian, Dominican, Jamaican, Filipinos, Pakistani, Guatemalan, Salvadoran, and other immigrants now are further transforming the Mid-Atlantic economy and culture.

The Mid-Atlantic is a relatively affluent region of the nation, having 43 of the 100 highest-income counties in the nation based on median household income and 33 of the top 100 based on per capita income. Most of the Mid-Atlantic states rank among the 15 highest-income states in the nation by median household income and per capita income.