The dean of the United States Senate is an informal term for the senator with the longest continuous service, regardless of party affiliation. This is not an official position within the Senate, although customarily the longest-serving member of the majority party serves as president pro tempore.

Thomas Hart Benton, nicknamed "Old Bullion", was an American politician, attorney, soldier, and longtime United States Senator from Missouri. A member of the Democratic Party, he was an architect and champion of westward expansion by the United States, a cause that became known as Manifest Destiny. Benton served in the Senate from 1821 to 1851, becoming the first member of that body to serve five terms.