Ruth Plumly Thompson (1891–1976) was an American writer of children's stories. She is best known for continuing the children's fantasy Land of Oz series as Royal Historian after L. Frank Baum died in 1919.
An avid reader of Baum's books and a lifelong children's writer, Thompson was born in Philadelphia, Pennsylvania and began her writing career in 1914 when she sold stories to newspapers and magazines. Between 1921 and 1939, she wrote one Oz book a year. They were all illustrated by John R. Neill, who had also illustrated Baum's Oz books, except for the first one, The Wonderful Wizard of Oz.
Thompson's contributions to the Oz series are lively and imaginative, featuring a wide range of colorful and unusual characters. However, one particular theme repeats over and over throughout her novels, with little variation. Typically in each of Thompson's Oz novels, a child (usually from America) and a supernatural companion (usually a talking animal), while traveling through Oz or one of the neighboring regions, find themselves in an obscure community where the inhabitants engage in a single activity. The inhabitants of this community then capture the travelers, and force them to participate in this same activity. Another major theme has elderly characters, most controversially, the Good Witch of the North, being restored to "marriageable" age, possibly because Thompson herself never married. She had a greater tendency toward the use of romantic love stories (which Baum usually avoided in his fairy tales, with about four exceptions), and characters such as the athletic Peter Brown gave a "boy's own" quality to some of her Oz books. She emphasized humor to a greater extent than Baum did, and always considered her work for children, whereas Baum saw no such restrictions on his intended audience.
Thompson also wrote promotional materials for the Reilly & Lee company — the short play A Day in Oz, and the short story "The Enchanted Tree of Oz." She also wrote Jean Gros's 1928 Oz marionette show, The Magical Land of Oz.
Thompson resigned from writing Oz books in 1939 and John R. Neill took over, writing three until his death in 1942. Thompson later wrote two more Oz books, which are not part of the "Famous Forty": Yankee in Oz (1972) and The Enchanted Island of Oz (1976).