Thomas Lanier Williams III (March 26, 1911 – February 25, 1983) was an American playwright and screenwriter. Along with contemporaries Eugene O'Neill and Arthur Miller, he is considered among the three foremost playwrights of 20th-century American drama.

Williams wrote his first play in 1930, but his work did not gain much traction until 1944 with the success of The Glass Menagerie. His next plays, including A Streetcar Named Desire (1947), Cat on a Hot Tin Roof (1955), Sweet Bird of Youth (1959), and The Night of the Iguana (1961), were also successful and widely acclaimed. With his later work, Williams attempted a new style that did not appeal as widely to audiences. His drama A Streetcar Named Desire is often numbered on short lists of the finest American plays of the 20th century alongside Eugene O'Neill's Long Day's Journey into Night and Arthur Miller's Death of a Salesman.

Much of Williams's work were adapted to film. He also wrote short stories, poetry, essays, and a volume of memoirs. In 1979, four years before his death, Williams was inducted into the American Theater Hall of Fame.