Alfred Abraham Knopf Sr. (September 12, 1892 – August 11, 1984) was an American publisher of the 20th century, and co-founder of Alfred A. Knopf, Inc. His contemporaries included the likes of Bennett Cerf and Donald Klopfer, and (of the previous generation) Frank Nelson Doubleday, J. Henry Harper and Henry Holt. Knopf paid special attention to the quality of printing, binding and design in his books, and earned a reputation as a purist in both content and presentation.

Knopf was born into a Jewish family in New York City. His father, Samuel Knopf, was an advertising executive and financial consultant, and his mother was Ida Japhe, a school teacher. Samuel Knopf was originally from Warsaw, Poland, but came to New York with his parents, where he worked his way up the directorship at a small mercantile bank. Alfred's mother, Ida, was from a Latvian Jewish family who settled in New York. For a time Knopf's parents lived in the Midwest and in Virginia. Ida committed suicide when Alfred was five years old and his sister Sophia was almost two. That same day, Alfred's father had filed for divorce in which he named Ida as an adulteress. His father later married Lillian Harris, who had a daughter, Bertha, from a previous marriage. With Lillian, Samuel had another son, Edwin H. Knopf, who worked for Alfred briefly, then became a film director and producer.

Alfred attended Columbia University, where he was a pre-law student and a member of the Peithologian Society (a debating and literary club) and the Boar's Head Society. He began to show an interest in publishing during his senior year, becoming advertising manager of an undergraduate magazine. His interest in publishing was allegedly fostered by a correspondence with British author John Galsworthy. Galsworthy was the subject of Knopf's senior thesis and after visiting Galsworthy in England, Knopf gave up his plans for a law career, and upon his return went into publishing.

Knopf was introduced to his future wife and business partner, Blanche Knopf, at a party at the Lawrence Athletic Club in 1911. Their relationship was built on their mutual interest in books. Blanche said of their relationship: "Alfred had realized I read books constantly and he had never met a girl who did ... I saw him and [all we did was] talk books, and nobody liked him—my family least of all. But I did, because I had someone to talk books to and we talked of making books...We decided we would get married and make books and publish them." Alfred and Blanche were married on April 4, 1916.

Knopf worked as a clerk at Doubleday (1912–1913), then as an editorial assistant to Mitchell Kennerley (1914).

Knopf, along with Blanche Knopf, founded the publishing house Alfred A. Knopf in 1915. The company initially emphasized European, especially Russian, literature, hence the choice of the borzoi as a colophon. At that time European literature was largely neglected by American publishers; Knopf published authors such as Simone de Beauvoir, Albert Camus, Joseph Conrad, E. M. Forster, Sigmund Freud, André Gide, Franz Kafka, D. H. Lawrence, Thomas Mann, W. Somerset Maugham, T. F. Powys, Wyndham Lewis and Jean-Paul Sartre. While Blanche was known as a superb editor, Alfred was always interested in more of the sales side than in editing.

Knopf also published many American authors, including Conrad Aiken, James Baldwin, James M. Cain, Theodore Dreiser, Shirley Ann Grau, Dashiell Hammett, Langston Hughes, Vachel Lindsay, H. L. Mencken, George Jean Nathan, John Updike, and Knopf's own favorite, Willa Cather. From 1924 to 1934, he published the famous literary magazine founded by Mencken and Nathan, The American Mercury. He often developed a personal friendship with his authors. Knopf's personal interest in the fields of history, sociology and science led to close friendships in the academic community with such noted historians as Richard Hofstadter, Arthur Schlesinger Jr. and Samuel Eliot Morison. A prominent Republican until Watergate, Knopf often drew legislators into lengthy correspondence by mail. He was also a member of the Peabody Awards Board of Jurors from 1940 to 1946.

Knopf himself was also an author. His writings include Some Random Recollections, Women, Publishing Then and Now, Portrait of a Publisher, Blanche W. Knopf: July 30, 1894–June 4, 1966, and Sixty Photographs.