Robert Stanton Baum (12 February 1886 – 21 April 1958) was the second of the four sons of L. Frank Baum and Maud Gage Baum. His father dedicated his 1901 novel The Master Key to Robert, and it is generally thought that Rob, the novel's protagonist, was based on Robert Baum. Two decades later, Baum dedicated his final Oz book, Glinda of Oz, to Robert.
R. S. Baum served in the U.S. Army Corps of Engineers in France during World War I. His "The Autobiography of Robert Stanton Baum" was published in The Baum Bugle, Christmas 1970 and Spring 1971.
He married Edna Ducker (1887–1968), who had been a neighbor of the Baum family at their summer home in Macatawa Park, Michigan, during the first decade of the twentieth century. They had three children, Robert Allison, Stanton, and Florence.