For an alternative meaning of the term free state, see Free state (government).

The free states of the United States existed in opposition to the slave states prior to the American Civil War. The term free state described a state in the antebellum United States in which slavery was either prohibited or eliminated over time. The Northeastern and Mid-Atlantic States, including Massachusetts, New York, Pennsylvania and New Jersey, had legally sanctioned slavery in the 17th, 18th and even part of the 19th centuries, but in the generation or two before the American Civil War, almost all slaves had been emancipated through a series of statutes.

The first U.S. region entirely free of slavery was the U.S. Midwest, which was ordained free under the Northwest Ordinance of 1787, passed just before the U.S. Constitution was ratified. The states created from this region—Ohio, Indiana, Michigan, Illinois and Wisconsin—were generally settled by New Englanders and American Revolutionary War veterans granted land there. Because this region was entirely free from its inception, and separated by the Ohio River from the South, where an expansion of legal slavery was pushing West, the concept of "free states" developed in constrast to "slave states" to the South. The rural Midwest, at one time in direct East-West rivalry with the Northeastern commercial states, realigned with the Northeastern states, newly free of slavery, and together created the amalgamation of states prohibiting slavery known in the context of the Civil War as the free states.

At the beginning of the American Civil War, the free states were as follows: Maine, Vermont, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Ohio, Indiana, Illinois, Michigan, Wisconsin, Minnesota, Iowa, Oregon and California.