William Graham Sumner (October 30, 1840 – April 12, 1910) was an American Episcopal priest, social scientist, and neoclassical liberal. He taught social sciences at Yale University, where he held the nation's first professorship in sociology and became one of the most influential teachers at any major school.

Sumner wrote extensively on the social sciences, penning numerous books and essays on ethics, American history, economic history, political theory, sociology, and anthropology. He supported laissez-faire economics, free markets, and the gold standard, in addition to coining the term "ethnocentrism" to identify the roots of imperialism, which he strongly opposed. As a spokesman against elitism, he was in favor of the "forgotten man" of the middle class—a term he coined. He had a prolonged influence on American conservatism.

Sumner wrote an autobiographical sketch for the fourth of the histories of the Class of 1863 Yale College. In 1925, the Rev. Harris E. Starr, class of 1910 Yale Department of Theology, published the first full-length biography of Sumner. A second full-length biography by Bruce Curtis was published in 1981.

Sumner was born in Paterson, New Jersey, on October 30, 1840. His father, Thomas Sumner, was born in England and immigrated to the United States in 1836. His mother, Sarah Graham, was also born in England. She was brought to the United States in 1825 by her parents. Sumner's mother died when he was eight.

In 1841, Sumner's father went prospecting as far west as Ohio, but came back east to New England and settled in Hartford, Connecticut, in about 1845. Sumner wrote about his high regard for his father: "His principles and habits of life were the best possible." Earlier in his life, Sumner said, he accepted from others "views and opinions" different from his father's. However, "at the present time," Sumner wrote, "in regard to those matters, I hold with him and not with the others." Sumner did not name the "matters."

Sumner was educated in the Hartford public schools. After graduation, he worked for two years as a clerk in a store before going to Yale College, graduating in 1863. Sumner achieved an impressive record at Yale as a scholar and orator. He was elected to Phi Beta Kappa society in his junior year and in his senior year to the secretive Skull and Bones society.

Sumner avoided being drafted to fight in the American Civil War by paying a "substitute" $250, given to him by a friend, to enlist for three years. This and money given to him by his father and friends allowed Sumner to go to Europe for further studies. He spent his first year in the University of Geneva studying Latin and Hebrew and the following two years in the University of Göttingen studying ancient languages, history and Biblical science. All told, in his formal education Sumner learned Hebrew, Greek, Latin, French, and German. In addition, after middle age he taught himself Dutch, Spanish, Portuguese, Italian, Russian, Polish, Danish, and Swedish.

In May 1866, he went to Oxford University to study theology. At Oxford, Henry Thomas Buckle planted the sociology seed in Sumner's mind. However, Herbert Spencer was to have the "dominating influence upon Sumner's thought."