Rufus King (March 24, 1755 – April 29, 1827) was an American Founding Father, lawyer, politician, and diplomat. He was a delegate from Massachusetts to the Continental Congress and the Philadelphia Convention and was one of the signers of the United States Constitution in 1787. After formation of the new Congress, he represented New York in the United States Senate. He emerged as a leading member of the Federalist Party and was the party's last presidential nominee during the 1816 presidential election.

The son of a prosperous Massachusetts merchant, King studied law before he volunteered for the militia during the American Revolutionary War. He won election to the Massachusetts General Court in 1783 and to the Congress of the Confederation the following year. At the 1787 Philadelphia Convention, he emerged as a leading nationalist and called for increased powers for the federal government. After the convention, King returned to Massachusetts, where he used his influence to help ratify the Constitution. At the urging of Alexander Hamilton, he then abandoned his law practice and moved to New York City.

He won election to represent New York in the United States Senate in 1789 and remained in office until 1796. That year, he accepted President George Washington's appointment to the position of Minister to Great Britain. Though King aligned with Hamilton's Federalist Party, the Democratic-Republican President Thomas Jefferson retained King's services after Jefferson's victory in the 1800 presidential election. King served as the Federalist vice-presidential candidate in the 1804 and 1808 elections and ran on an unsuccessful ticket with Charles Cotesworth Pinckney of South Carolina. Though most Federalists supported the Democratic-Republican DeWitt Clinton in the 1812 presidential election, King, without the support of his party, won the few votes of the Federalists who were unwilling to support Clinton's candidacy. In 1813, King returned to the Senate and remained in office until 1825.

King, the de facto Federalist nominee for president in 1816, lost in a landslide to James Monroe. The Federalist Party became defunct at the national level after 1816, and King was the last presidential nominee whom the party fielded. Nonetheless, King was able to remain in the Senate until 1825, which made him the last Federalist senator because of a split in the New York Democratic-Republican Party. King then accepted President John Quincy Adams's appointment to serve another term as ambassador to Great Britain, but ill health forced King to retire from public life, and he died in 1827. King had five children who lived to adulthood, and he has had numerous notable descendants.

King was born on March 24, 1755, in Scarborough, which was then part of Massachusetts but is now in Maine. He was a son of Isabella (Bragdon) and Richard King, a prosperous farmer, merchant, lumberman, and sea captain who had settled at Dunstan Landing in Scarborough, near Portland, Maine, and had made a modest fortune by the time Rufus was born. His financial success aroused the jealousy of his neighbors, and when the unpopular Stamp Act 1765 was imposed, a mob ransacked his house and destroyed most of the furniture. Nobody was punished, and the next year, the mob burned down his barn. John Adams once referred to that moment in discussing limitations of the "mob" for the Constitutional Convention and wrote a letter to his wife, Abigail, in which he described the scene:

I am engaged in a famous Cause: The Cause of King, of Scarborough vs. a Mob, that broke into his House, and rifled his Papers, and terrifyed him, his Wife, Children and Servants in the Night. The Terror, and Distress, the Distraction and Horror of this Family cannot be described by Words or painted upon Canvass. It is enough to move a Statue, to melt an Heart of Stone, to read the Story....

Richard King was a Loyalist during the American Revolutionary War, but all his sons became Patriots.

King attended Dummer Academy (now The Governor's Academy) from the age of twelve in South Byfield, Massachusetts. Later, he attended Harvard College, where he graduated in 1777. He began to read law under Theophilus Parsons, but his studies were interrupted in 1778, when King volunteered for militia duty during the American Revolutionary War. Appointed a major, he served as an aide to General John Sullivan during the Battle of Rhode Island. After the campaign, King returned to his apprenticeship under Parsons. He was admitted to the bar in 1780 and began a legal practice in Newburyport, Massachusetts.