Agnes Ruby Boulton (September 19, 1893 – November 25, 1968) was a British-born American pulp magazine writer in the 1910s, later the wife of Eugene O'Neill.

[7] During the early years of the marriage, Boulton modified her writing and had two stories published by The Smart Set, an important magazine co-edited by H. L. Mencken and George Jean Nathan.

[9] The Boulton/O'Neill marriage has been studied and written about by William Davies King, professor of theater at UC Santa Barbara, in "Another Part of a Long Story: Literary Traces of Eugene O'Neill and Agnes Boulton" (Michigan 2010).

[9] Her daughter, Oona O'Neill, married Charlie Chaplin in 1943 at the age of 18 (he was 54), and moved to Switzerland with him nine years later, renouncing her American citizenship.

Boulton published a novel, The Road Is Before Us, in 1944, and a memoir of the first two years of her marriage to O'Neill in 1958, entitled Part of a Long Story.