Ohio is a Midwestern state of the United States of America. Part of the Great Lakes region, Ohio has long been a cultural and geographical crossroads. At the time of European contact and in the years which followed, Native Americans in today's Ohio included the Iroquois, Miamis, and Wyandots. Beginning in the 1700s, the area was settled by people from New England, the Middle States, Appalachia, and the upper south.

Prior to 1984, the United States Census Bureau considered Ohio part of the North Central Region. That region was renamed "Midwest" and split into two divisions. Ohio is now in the East North Central States division. Ohio also has the highest density of population of any state not on the Atlantic Ocean.

Ohio was the first state admitted to the Union under the Northwest Ordinance. Its U.S. postal abbreviation is OH; its old-style abbreviation was O. Natives of Ohio are known as Ohioans.

After the so-called Beaver Wars, the powerful Iroquois confederation of the New York-area claimed much of the Ohio country as a hunting and, probably most importantly, a beaver-trapping ground. After the devastation of epidemics and war in the mid-1600s, which had largely emptied the Ohio country of indigenous people by the mid-to-late seventeenth century, the land gradually became repopulated by the mostly Algonquian-speaking descendants of its ancient inhabitants, that is, descendants of the Adena, Hopewell, and Mississippian cultures. Many of these Ohio-country nations were multi-ethnic and sometimes multi-linguistic societies born out of the earlier devastation brought about by disease, subsequent social instability, Iroquois. They subsisted on agriculture (corn, sunflowers, beans, etc.) supplemented by seasonal hunts. By the 1650s they were very much part of a larger global economy brought about by fur trade.

The indigenous nations to inhabit Ohio in the historical period (most clearly after 1700), included the Miamis (a large confederation), Wyandots (made up of refugees, especially from the fractured Huron confederacy), Delawares (pushed west from their historic homeland in New Jersey), Shawnees (also pushed west, although they may be descended from the Fort Ancient people of Ohio), Ottawas (more commonly associated with the upper Great Lakes region), Mingos (like the Wyandot, a recently-formed composite of refugees from Iroquois and other societies), and Eries (gradually absorbed into the new, multi-ethnic "republics," namely the Wyandot).

Ohio country was also the site of Indian massacres, such as the Yellow Creek Massacre (Chief Logan) and Gnadenhutten.

During the 18th century, the French set up a system of trading posts to control the fur trade in the region. In 1754, France and Great Britain fought a war known in the United States as the French and Indian War. As a result of the Treaty of Paris, the French ceded control of Ohio and the rest of the Old Northwest to Great Britain. Pontiac's Rebellion in the 1760s challenged British military control, which ended with the American victory in the American Revolution. In the Treaty of Paris in 1783 Britain ceded all claims to Ohio to the United States.

The United States created the Northwest Territory under the Northwest Ordinance of 1787, slavery was not permitted. Settlement began with the founding of Marietta by the Ohio Company of Associates, which had been formed by a group of American Revolutionary War veterans. Following the Ohio Company, the Miami Company (also referred to as the "Symmes Purchase") claimed the southwestern section and the Connecticut Land Company surveyed and settled the Connecticut Western Reserve in present-day Northeast Ohio. The old Northwest Territory originally included areas that had previously been known as Ohio Country and Illinois Country. As Ohio prepared for statehood, Indiana Territory was created, reducing the Northwest Territory to approximately the size of present-day Ohio plus the eastern half of the Lower Peninsula of Michigan and the eastern tip of the Upper Peninsula.

Under the Northwest Ordinance, any of the states to be formed out of the Northwest Territory would be admitted as a state once the population exceeded 60,000. Although Ohio's population numbered only 45,000 in December 1801, Congress determined that the population was growing rapidly and Ohio could begin the path to statehood with the assumption that it would exceed 60,000 residents by the time it would become a state. On February 19, 1803, President Jefferson signed an act of Congress that approved Ohio's boundaries and constitution. However, Congress never passed a resolution formally admitting Ohio as the 17th state. The current custom of Congress declaring an official date of statehood did not begin until 1812, with Louisiana's admission as the 18th state. Although no formal resolution of admission was required, when the oversight was discovered in 1953, Ohio congressman George H. Bender introduced a bill in Congress to admit Ohio to the Union retroactive to March 1, 1803. At a special session at the old state capital in Chillicothe, the Ohio state legislature approved a new petition for statehood that was delivered to Washington, D.C. on horseback. On August 7, 1953 (the year of Ohio's 150th anniversary), President Eisenhower signed an act that officially declared March 1, 1803 the date of Ohio's admittance into the Union.

Eight U.S. presidents hailed from Ohio at the time of their elections, giving rise to the nickname "Mother of Presidents", a sobriquet it shares with Virginia. Seven presidents were born in Ohio, making it second to Virginia's eight, but Virginia-born William Henry Harrison and his grandson, Benjamin Harrison, (who also lived part of his adult life in Indiana) settled in, led their political careers from and/or were buried in North Bend, Ohio on the family compound, founded by William's father-in-law John Cleves Symmes.

In 1835, Ohio fought a mostly bloodless boundary war with Michigan over the Toledo Strip known as the Toledo War. Congress intervened and, as a condition for admittance as a state of the Union, Michigan was forced to accept the western two-thirds of the Upper Peninsula, in addition to the eastern third that was already part of the state, in exchange for giving up its claim to the Toledo Strip. (A war between two states may be unusual, but the Toledo War is not unique; Pennsylvania and Maryland fought Cresap's War over a border dispute a century earlier.)

Ohio's central position and its population gave it an important place during the Civil War, and the Ohio River was a vital artery for troop and supply movements, as were Ohio's railroads. At the end of the Civil War, three top Union generals were all from Ohio: Grant, Sherman, and Sheridan. Ohio also contributed more soldiers per-capita than any other state in the Union.

In 1912 a Constitutional Convention was held with Charles B. Galbreath as Secretary. The result reflected the concerns of the Progressive Era. It introduced the initiative and the referendum, allowed the General Assembly to put questions on the ballot for the people to ratify laws and constitutional amendments originating in the Legislature as well. Under the Jeffersonian principle that laws should be reviewed once a generation, the constitution provided for a recurring question to appear on Ohio's general election ballots every 20 years. The question asks whether a new convention is required. Although the question has appeared in 1932, 1952, 1972, and 1992, it has never been approved. Instead constitutional amendments have been proposed by petition to the legislature hundreds of times and adopted in a majority of cases.