Margaret Ayer Barnes (April 8, 1886, Chicago, Illinois – October 25, 1967, Cambridge, Massachusetts) was an American playwright, novelist, and short-story writer.
She attended Bryn Mawr College, where she earned an A.B. degree in 1907. In 1936, she received an honorary degree in Doctor of Letters from Oglethorpe University. She married Cecil Barnes in 1910, and had three sons, Cecil Jr., Edward Larrabee and Benjamin Ayer. In 1920, Barnes was elected alumnae director of Bryn Mawr and served three years. As director, she helped to organize the Bryn Mawr Summer School for Women Workers in Industry, which offered an alternative educational program for women workers within a traditional institution. Consisting mainly of young, single immigrant women with little to no academic background, the summer program offered courses in progressive education, liberal arts and economics. Women in the program were encouraged to develop confidence as speakers, writers and leaders in the workplace.
In 1926, at age 40, she broke her back in a traffic accident, and, with the encouragement of friend and playwright Edward Sheldon, took up writing as a way to occupy her time. Between 1926 and 1930 she wrote several short stories and three plays, including an adaptation of Edith Wharton's novel The Age of Innocence. In 1931 she won the Pulitzer Prize for her first novel, Years of Grace.
A 1936 lawsuit against Metro-Goldwyn-Mayer for copyright infringement claimed that the script MGM used for the motion picture Letty Lynton (1932) plagiarized material from the play Dishonored Lady by Edward Sheldon and Barnes. The film is still unavailable today because of this lawsuit.
Barnes was the wife of a prominent Chicago attorney. Her older sister was suffragette and fellow author Janet Ayer Fairbank (1878–1951), and her niece Janet Fairbank (1903–1947) was a well-known operatic singer. Her son was the noted architect Edward Larrabee Barnes (1915-2004).