The president pro tempore of the United States Senate (often shortened to president pro tem) is the second-highest-ranking official of the United States Senate. Article One, Section Three of the United States Constitution provides that the vice president of the United States is the president of the Senate (despite not being a senator), and mandates that the Senate must choose a president pro tempore to act in the vice president's absence. Unlike the vice president, the president pro tempore is an elected member of the Senate, able to speak or vote on any issue. Selected by the Senate at large, usually by a resolution which is adopted by unanimous consent without a formal vote, the president pro tempore has enjoyed many privileges and some limited powers. During the vice president's absence, the president pro tempore is empowered to preside over Senate sessions. Except when necessary or to highlight important votes, the vice president and the president pro tempore rarely preside; instead, the duty of presiding officer is rotated among junior U.S. senators of the majority party to give them experience in parliamentary procedure.Since 1890, the most senior U.S. senator in the majority party has generally been chosen to be president pro tempore and holds the office continuously until the election of another. This tradition has been observed without interruption since 1949. Since the enactment of the current Presidential Succession Act in 1947, 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 and ahead of the secretary of state.The current president pro tempore of the Senate is Patrick Leahy of Vermont. He was sworn in on January 20, 2021, during the 117th Congress, when the Democratic Party gained control of the Senate. He previously held the position from 2012 to 2015.

The United States Senate is the upper chamber of the United States Congress, with the House of Representatives being the lower chamber. Together they compose the national bicameral legislature of the United States.

The composition and powers of the Senate are established by Article One of the United States Constitution. The Senate is composed of senators, each of whom represents a single state in its entirety. Each state is equally represented by two senators who serve staggered terms of six years. There are currently 100 senators representing the 50 states. The vice president of the United States serves as presiding officer and president of the Senate by virtue of that office, and has a vote only if the senators are equally divided. In the vice president's absence, the president pro tempore, who is traditionally the senior member of the party holding a majority of seats, presides over the Senate.

As the upper chamber of Congress, the Senate has several powers of advice and consent which are unique to it. These include the approval of treaties, and the confirmation of Cabinet secretaries, federal judges (including Federal Supreme Court justices), flag officers, regulatory officials, ambassadors, other federal executive officials and federal uniformed officers. If no candidate receives a majority of electors for vice president, the duty falls to the Senate to elect one of the top two recipients of electors for that office. The Senate conducts trials of those impeached by the House.

The Senate is widely considered both a more deliberative and more prestigious body than the House of Representatives due to its longer terms, smaller size, and statewide constituencies, which historically led to a more collegial and less partisan atmosphere.From 1789 to 1913, senators were appointed by legislatures of the states they represented. They are now elected by popular vote following the ratification of the Seventeenth Amendment in 1913. In the early 1920s, the practice of majority and minority parties electing their floor leaders began. The Senate's legislative and executive business is managed and scheduled by the Senate majority leader.

The Senate chamber is located in the north wing of the Capitol Building in Washington, D.C.