John Cornelius Stennis was an American politician who served as a U.S. senator from the state of Mississippi. He was a Democrat who served in the Senate for over 41 years, becoming its most senior member for his last eight years. He retired from the Senate in 1989, and is, to date, the last Democrat to have been a U.S. senator from Mississippi. Furthermore, at the time of his retirement, Stennis was the last senator to have served during the presidency of Harry S. Truman.

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.