Lionel Mordecai Trilling (July 4, 1905 – November 5, 1975) was an American literary critic, short story writer, essayist, and teacher. One of the leading U.S. critics of the 20th century, he analyzed the contemporary cultural, social, and political implications of literature. He and his wife, Diana Trilling (née Rubin), were members of the New York Intellectuals and contributors to the Partisan Review.

Lionel Mordecai Trilling was born in Queens, New York, the son of Fannie (née Cohen), who was from London, and David Trilling, a tailor from Bialystok in Poland. His family was Jewish. In 1921, he graduated from DeWitt Clinton High School, and, at age 16, entered Columbia University, beginning a lifelong association with the university. He joined the Boar's Head Society and wrote for the Morningside literary journal. In 1925, he graduated from Columbia College, and in 1926 he earned a master's degree at the university (his master's essay was titled Theodore Edward Hook: his life and work). He then taught at the University of Wisconsin–Madison and at Hunter College.

In 1929, he married Diana Rubin, and the two began a lifelong literary partnership. In 1932, he returned to Columbia to pursue his doctoral degree in English literature and to teach literature. He earned his doctorate in 1938 with a dissertation about Matthew Arnold that he later published. He was promoted to assistant professor the next year, becoming Columbia's first tenured Jewish professor in its English department. He was promoted to full professor in 1948.

Trilling became the George Edward Woodberry Professor of Literature and Criticism in 1965. He was a popular instructor and for 30 years taught Columbia's Colloquium on Important Books, a course about the relationship between literature and cultural history, with Jacques Barzun. His students included Lucien Carr, Jack Kerouac, Donald M. Friedman, Allen Ginsberg, Eugene Goodheart, Steven Marcus, John Hollander, Richard Howard, Cynthia Ozick, Carolyn Gold Heilbrun, George Stade, David Lehman, Leon Wieseltier, Louis Menand, Robert Leonard Moore, and Norman Podhoretz.

Trilling was the George Eastman Visiting Professor at the University of Oxford from 1963 to 1965 and Charles Eliot Norton Professor of Poetry at Harvard University for academic year 1969–70. In 1972, the National Endowment for the Humanities selected him to deliver the first Jefferson Lecture in the Humanities, described as "the highest honor the federal government confers for distinguished intellectual achievement in the humanities." Trilling was a senior Fellow of the Kenyon School of English and subsequently a senior Fellow of the Indiana School of Letters. He held honorary degrees from Trinity College, Harvard University, and Case Western Reserve University and memberships in the American Academy of Arts and Sciences and the American Academy of Arts and Letters. He also served on the boards of The Kenyon Review and Partisan Review.

Trilling, a longtime heavy smoker, died of pancreatic cancer in 1975. He was survived by his wife and son, James Trilling, an art historian who served as a curator at the George Washington University Museum and Textile Museum. His nephew Billy Cross is a musician residing in Denmark.

In 1937, Trilling joined the recently revived magazine Partisan Review, a Marxist but anti-Stalinist journal founded by William Philips and Philip Rahv in 1934.

The Partisan Review was associated with the New York Intellectuals—Trilling, Diana Trilling, Lionel Abel, Hannah Arendt, William Barrett, Daniel Bell, Saul Bellow, Richard Thomas Chase, F. W. Dupee, Leslie Fiedler, Paul Goodman, Clement Greenberg, Elizabeth Hardwick, Irving Howe, Alfred Kazin, Hilton Kramer, Steven Marcus, Mary McCarthy, Dwight Macdonald, William Phillips, Norman Podhoretz, Harold Rosenberg, Isaac Rosenfeld, Delmore Schwartz, and Susan Sontag—who emphasized the influence of history and culture on authors and literature. The New York Intellectuals distanced themselves from the New Critics.