The Frontier Thesis or Turner Thesis, is the argument advanced by historian Frederick Jackson Turner in 1893 that the origin of the distinctive egalitarian, democratic, aggressive, and innovative features of the American character has been the American frontier experience. He stressed the process—the moving frontier line—and the impact it had on pioneers going through the process. In the thesis, the frontier established liberty by releasing Americans from European mind-sets and ending prior customs of the 19th century. Turner first announced his thesis in a paper entitled "The Significance of the Frontier in American History", delivered to the American Historical Association in 1893 in Chicago.

Other historians had begun to explore the meaning of the frontier, such as Theodore Roosevelt, who had a different theory. Roosevelt argued that the battles between the trans-Appalachian pioneers and the Indians in the "Winning of the West" had forged a new people, the American race.

Turner set up an evolutionary model (he had studied evolution with a leading geologist, Thomas Chrowder Chamberlin), using the time dimension of American history, and the geographical space of the land that became the United States. The first settlers who arrived on the east coast in the 17th century acted and thought like Europeans. They encountered environmental challenges that were different from those they had known in Europe. Most important was the presence of uncultivated arable land. They adapted to the new environment in certain ways — the cumulative effect of these adaptations was Americanization. According to Turner, the forging of the unique and rugged American identity had to occur precisely at the juncture between the civilization of settlement and the savagery of wilderness. The dynamic of these oppositional conditions engendered a process by which citizens were made, citizens with the power to tame the wild and upon whom the wild had conferred strength and individuality...