Lucius Quintus Cincinnatus Lamar (September 17, 1825 – January 23, 1893) was an American politician and jurist from Mississippi. A United States Representative and Senator, he also served as United States Secretary of the Interior in the first administration of President Grover Cleveland, as well as an Associate Justice of the U.S. Supreme Court. During the American Civil War, he participated in the Confederate government, briefly leading a volunteer regiment, and serving on diplomatic missions to Europe. James Longstreet was his cousin.