Louis Hartz (April 8, 1919 – January 20, 1986) was an American political scientist, historian, and a professor at Harvard University, where he taught from 1942 until 1974. Hartz's teaching and various writings—books and articles—have had an important influence on American political theory and comparative history.

Hartz was born in Youngstown, Ohio, on April 8, 1919, the son of Russian Jewish immigrants. He grew up in Omaha, Nebraska, where he attended Technical High School in Omaha. He attended Harvard University, financed partly by a scholarship from the Omaha World-Herald, and graduated in 1940. He then spent a year traveling abroad on a fellowship. In 1942, Harts returned to Harvard, where he became a teaching fellow and earned his doctorate in 1946.

In 1956, Hartz became a full professor of government at Harvard University, where he was known as a talented and charismatic professor.

In 1955, Hartz authored and published his classic book The Liberal Tradition in America, in which he sought to explain the absence of ideologies in U.S. history. Hartz argued that American politics is guided by an enduring and underlying Lockean liberal consensus, which has shaped and narrowed the landscape of possibilities for U.S. political thought and behavior. Hartz attributed this triumph of the liberal worldview in the United States to the lack of a feudal past, which accounts for the absence of a struggle to overcome a conservative internal order, its vast resources and open space, and its liberal values introduced by its original settlers, who represented a narrow middle class component of European society.

Hartz also explained the rejection of socialism in the United States, which he attributed to Americans' widespread and generally consensual acceptance of classic liberalism and served as the major barrier to socialism in the nation.

Hartz edited and wrote substantial sections of The Founding of New Societies, published in 1964, in which he developed and expanded upon his “fragment thesis.” Hartz developed this thesis from the idea that those nations which originated as settler colonies are “fragments” of the original European nation that founded them. Hartz called them fragments because these colonies, in a sense, froze the class structure and underlying ideology prevalent in the mother country at the time of their foundation and did not experience the further evolution experienced in Europe. He considered Latin America and French Canada to be fragments of feudal Europe; the United States, English Canada, and Dutch South Africa to be liberal fragments; and Australia and English South Africa to be "radical" fragments (incorporating the nonsocialist working class radicalism of Britain in the early 19th century).

Hartz led a normal life until a sudden and unexplained emotional disturbance changed his entire personality in 1971. He refused all medical help. He divorced in 1972, rejected all his friends, and feuded intensely with Harvard students, faculty, and administrators. In 1974, he resigned, but he continued to utilize his scholarly skills and pursue his interests. Hartz spent his final years in London, New Delhi, New York City, and then Istanbul, where he died of an epileptic seizure in January 1986.

In 1956, the American Political Science Association awarded Hartz the Woodrow Wilson Prize for The Liberal Tradition in America. In 1977, he was awarded the association's Lippincott Prize, which honors scholarly works of enduring importance. The book remains a key text in graduate-level curricula in political science and is considered one of the most extensive overviews of the influence of the liberal tradition on American politics.