Selden P. 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 Washington University Law School, 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 circuit court of St. Louis, and was a captain in the Missouri Home Guard and chairman of the draft board in 1917-1918.
Spencer was elected, on November 5, 1918, as a Republican to the U.S. Senate to fill the vacancy caused by the death of William J. Stone; he was reelected in 1920 and served from November 6, 1918, until his death. While in the Senate, he was chairman of the Committee on Claims (Sixty-sixth and Sixty-seventh Congresses) and a member of the Committee on Indian Affairs (Sixty-seventh Congress) and the Committee on Privileges and Elections (Sixty-seventh through Sixty-ninth Congresses). He died at Walter Reed Hospital, Washington, D.C., in 1925; interment was in Bellefontaine Cemetery, St. Louis.