Robert Carlyle Byrd (born November 20, 1917) is the senior United States Senator from West Virginia, and a member and former Senate Leader of the Democratic Party. Byrd has been a Senator since January 3, 1959 and is the longest-serving member in the Senate's history; he has been the Dean of the Senate since 2003. He is also the oldest current member of the United States Congress, and is the first person to serve uninterrupted for half a century as a U.S. senator.

Byrd is President pro tempore of the United States Senate, a position that puts him third in the line of presidential succession, behind Vice President Joe Biden and House Speaker Nancy Pelosi. He also held this post previously from 1989 to 1995, briefly in January 2001, and from June 2001 to January 2003. In this role, Sen. Byrd signs bills passed by Congress before they are sent to the president to be signed into law or vetoed.

Byrd holds a wide variety of both liberal and conservative political views. A lifelong Democrat, Byrd did not leave the party as its views shifted from social conservatism to social liberalism. He has also held many leadership positions: Senate Conference Secretary, Majority Whip, Minority Leader and twice Majority Leader. He is the only former party leader currently in the Senate.