The Northwest Territory, also known as the Old Northwest and formally known as the Territory Northwest of the River Ohio, was formed from part of the unorganized western territory of the United States after the American Revolution. Established in 1787 by the Congress of the Confederation through the Northwest Ordinance, it was the nation's first post-colonial organized incorporated territory.

At the time of its creation, the territory included all the land west of Pennsylvania, northwest of the Ohio River and east of the Mississippi River below the Great Lakes, and what later became known as the Boundary Waters. The region was ceded to the United States in the Treaty of Paris of 1783. Throughout the Revolutionary War, the region was part of the British Province of Quebec and the western theater of the war. It spanned all or large parts of six eventual U.S. states (Ohio, Indiana, Illinois, Michigan, Wisconsin, and the northeastern part of Minnesota). Reduced to present-day Ohio, and some additional lands north and east on July 4, 1800, it ceased to exist March 1, 1803, when the southeastern portion of the territory was admitted to the Union as the state of Ohio, and the remainder attached to Indiana Territory.

Initially, the territory was governed by martial law under a governor and three judges. As population increased, a legislature was formed as were a succession of counties, eventually totaling thirteen. At the time of its creation, the land within the territory was largely undisturbed by urban development. It was also home to several Native American cultures, including the Delaware, Miami, Potawatomi, Shawnee, and others. There were a handful of French colonial settlements remaining, plus Clarksville at the Falls of the Ohio. By the time of the territory's dissolution, there were dozens of towns and settlements, a few with thousands of settlers, chiefly along the Ohio and Miami Rivers and the south shore of Lake Erie in Ohio. Conflicts between settlers and Native American inhabitants of the Territory resulted in the Northwest Indian War culminating in General "Mad" Anthony Wayne's victory at Battle of Fallen Timbers in 1794. The subsequent Treaty of Greenville in 1795 opened the way for settlement particularly in southern and western Ohio.

The Northwest Territory included all the then-owned land of the United States west of Pennsylvania, east of the Mississippi River, and northwest of the Ohio River. It incorporated most of the former Ohio Country except a portion in western Pennsylvania, and eastern Illinois Country. It covered all of the modern states of Ohio, Indiana, Illinois, Michigan, and Wisconsin, as well as the northeastern part of Minnesota.

Lands west of the Mississippi River were the Louisiana Province of New Spain, formerly New France, acquired by the United States in 1803 by the Louisiana Purchase. Lands north of the Great Lakes were part of the British Province of Upper Canada. Lands south of the Ohio River constituted Kentucky County, Virginia, admitted to the union as the state of Kentucky in 1792. The area included more than 300,000 square miles (780,000 km²) and comprised about 1/3 of the land area of the United States at the time of its creation.

It was inhabited by about 45,000 Native Americans and 4,000 non-native traders, mostly of French Canadian, British or Irish descent. Among the tribes inhabiting the region were the Shawnee, Delaware, Miami, Wyandot, Ojibwa, Ottawa and Potawatomi. Notably, the Miami tribal capital along with British trading posts was at Kekionga, at the site of present-day Fort Wayne, Indiana. Neutralizing Kekionga became the focus of the Northwest Indian War, the driving events in the early evolution of the territory.

Integration of the Northwest Territory into a political unit, and settlement, depended on three factors: relinquishment by the British, extinguishment of states' claims west of the Appalachians, and usurpation or purchase of lands from the Native Americans. These objectives were accomplished correspondingly by the American Revolutionary War, provisions in the Articles of Confederation, and various treaties preceding the Northwest Indian War including Treaty of Fort Stanwix (1784) and Treaty of Fort McIntosh (1785). The treaty process extended well beyond the War and existence of the Territory as a political entity.

European exploration of the region began with French-Canadian voyageurs in the 17th century, followed by French missionaries and French fur traders. French-Canadian explorer Jean Nicolet was the first recorded European entrant into the region, landing in 1634 at the current site of Green Bay, Wisconsin (although Étienne Brûlé is stated by some sources as having explored Lake Superior and possibly inland Wisconsin in 1622). The French exercised control from widely separate posts in the region, which they claimed as New France; among these was the post at Fort Detroit, founded in 1701. France ceded the territory to the Kingdom of Great Britain as part of the Indian Reserve in the 1763 Treaty of Paris, after being defeated in the French and Indian War.