The President pro tempore (pron.: /ˌproʊ ˈtɛmpəriː/ or /ˌproʊ ˈtɛmpəreɪ/), also president pro tem, is the second-highest-ranking official of the United States Senate. The United States Constitution states that the Vice President of the United States is the President of the Senate, despite not being a senator, and that the Senate must choose a president pro tempore. By a long-standing tradition which has been observed consistently since the 81st Congress (January 1949 – January 1951), the president pro tempore is the most senior senator in the majority party.

During the Vice President's absence, the president pro tempore is empowered to preside over Senate sessions but usually appoints another senator to do so. In practice, neither the Vice President nor the President pro tempore usually presides; instead, the duty of presiding officer is rotated among junior senators of the majority party to give them experience in parliamentary procedure.

The president pro tempore is third in the line of succession to the presidency, after the Vice President and the Speaker of the House of Representatives.

United States presidential line of succession
Preceded by
Speaker of the House of Representatives
3rd in line Succeeded by
Secretary of State