Long Day's Journey into Night is a play in four acts written by American playwright Eugene O'Neill in 1939–1941 and first published posthumously in 1956. It is widely regarded as his magnum opus and one of the great American plays of the 20th century. It premiered in Sweden in February 1956 and then opened on Broadway in November 1956, winning the Tony Award for Best Play. O'Neill received the 1957 Pulitzer Prize for Drama posthumously for Long Day's Journey into Night. The work is openly autobiographical in nature. The "long day" in the title refers to the setting of the play, which takes place during one day.

Eugene Gladstone O'Neill was an American playwright. His poetically titled plays were among the first to introduce into the U.S. the drama techniques of realism, earlier associated with Chekhov, Ibsen, and Strindberg. The tragedy Long Day's Journey into Night is often included on lists of the finest U.S. plays in the 20th century, alongside Tennessee Williams's A Streetcar Named Desire and Arthur Miller's Death of a Salesman. He was awarded the 1936 Nobel Prize in Literature. O'Neill is also the only playwright to win four Pulitzer Prizes for Drama.