Rufus King (March 24, 1755 – April 29, 1827) is an American lawyer, politician, and diplomat.

He is a delegate for Massachusetts to the Continental Congress and the Philadelphia Convention and is one of the signers of the United States Constitution in 1787.

After formation of the new Congress he represents New York in the United States Senate.

He emerges as a leading member of the Federalist Party, serving as the party's last presidential nominee in the 1816 presidential election.

The son of a prosperous Massachusetts merchant, King studies law before volunteering for the militia in the American Revolutionary War.

He wins election to the Massachusetts General Court in 1783 and to the Congress of the Confederation the following year.

At the 1787 Philadelphia Convention, he emerges as a leading nationalist, calling for increased powers for the federal government.

After the convention, King returns to Massachusetts, where he uses his influence to help win ratification of the new Constitution.

At the urging of Alexander Hamilton, he then abandons his law practice and moves to New York City.

He wins election to represent New York in the United States Senate in 1789, remaining in office until 1796.

In this year, he accepts President George Washington's appointment to the position of Minister to Britain.

Though King aligned with Hamilton's Federalists, Democratic-Republican President Thomas Jefferson retains his services after Jefferson's victory in the 1800 presidential election.

King serves as the Federalist vice presidential candidate in the 1804 and 1808 elections, running on an unsuccessful ticket with Charles Cotesworth Pinckney of South Carolina.

Though most Federalists support Democratic-Republican DeWitt Clinton in the 1812 presidential election, King, without the support of his party, wins little votes of those Federalists who are unwilling to support Clinton's candidacy

In 1813, he returns to the Senate and remains in office until 1825.

King is the informal de facto Federalist nominee for president in 1816, losing in a landslide to James Monroe.

The Federalist Party becomes defunct at the national level after 1816, and King is the last presidential nominee the party fields.

Nonetheless, King is able to remain in the Senate until 1825 due to a split in the New York Democratic-Republican Party.

After this, King accepts John Quincy Adams's appointment to serve another term as ambassador to Britain, but ill health forces King to retire from public life, and he dies in 1827.

King has five children who live to adulthood, and he has numerous notable descendants.