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.

The president pro tempore of the United States Senate is the second-highest-ranking official of the United States Senate, after the vice president. According to Article One, Section Three of the United States Constitution, the vice president of the United States is the president of the Senate, and the Senate must choose a president pro tempore to act in the vice president's absence.