The Frontier Thesis, also known as Turner's Thesis or American frontierism, is the argument advanced by historian Frederick Jackson Turner in 1893 that the settlement and colonization of the rugged American frontier was decisive in forming the culture of American democracy and distinguishing it from European nations. He stressed the process of "winning a wilderness" to extend the frontier line further for U.S. colonization, and the impact this had on pioneer culture and character. Turner's text takes the ideas behind Manifest Destiny and uses them to explain how American culture came to be. The features of this unique American culture included democracy, egalitarianism, uninterest in bourgeois or high culture, and an ever-present potential for violence. "American democracy was born of no theorist's dream; it was not carried in the Susan Constant to Virginia, nor in the Mayflower to Plymouth. It came out of the American forest, and it gained new strength each time it touched a new frontier," wrote Turner.

Frederick Jackson Turner was an American historian during the early 20th century, based at the University of Wisconsin-Madison until 1910, and then Harvard University. He was known primarily for his frontier thesis. He trained many PhDs who went on to become well-known historians. He promoted interdisciplinary and quantitative methods, often with an emphasis on the Midwestern United States.