Selden Palmer Spencer (September 16, 1862 May 16, 1925) was a United States Senator from Missouri. Born in Erie, Pennsylvania, he attended the public schools there and graduated from Yale College in 1884 and from the law school of Washington University in St. Louis, Missouri, in 1886. He was admitted to the bar, commencing practice in St. Louis; he was a professor of medical jurisprudence in the Missouri Medical College at St. Louis in 1886, and was a member of the Missouri House of Representatives in 1895-1896. From 1897 to 1903 he was a judge of the United States circuit court of appeals at St. Louis. He and future governor Forrest Donnell formed the law firm of Spencer & Donnell. He was a captain in the Missouri Home Guard and a chairman of a draft board in 1917-1918.