William L. Marcy, American lawyer, judge, and politician, 21st United States Secretary of State (d. 1857)
William Learned Marcy (December 12, 1786 – July 4, 1857) was an American lawyer, politician, and judge who served as U.S. Senator, Governor of New York, U.S. Secretary of War and U.S. Secretary of State. In the latter office, he negotiated the Gadsden Purchase, the last major acquisition of land in the continental United States.

He was born in what is now Southbridge, Massachusetts (originally part of Sturbridge as Southbridge was only incorporated as a town in 1816), Marcy established a legal practice in Troy, New York after graduating from Brown University. He fought in the War of 1812, as an ensign, first lieutenant and captain in the 155th New York Infantry Regiment. Politically, he aligned with the Bucktail faction of the Democratic-Republican Party and became a leading member of the Albany Regency. As the Democratic-Republicans fractured in the 1820s, he became a member of the Democratic Party. Between 1821 and 1831, he successively served as Adjutant General of New York, New York State Comptroller, and as an associate justice of the New York Supreme Court. In 1831, the New York legislature elected Marcy to the U.S. Senate, and he held that position until 1833, when he became the Governor of New York. He served three terms as governor until his defeat in 1838 by the Whig nominee, William Seward.

He served as Secretary of War under James K. Polk from 1845 to 1849, overseeing the Mexican–American War. After leaving the Polk administration, he resumed the practice of law and became a leader of the "Soft" Hunker faction of the New York Democratic Party. He returned to the Cabinet in 1853, serving as Secretary of State under Franklin Pierce. In this role, he resolved a dispute about the status of U.S. immigrants abroad and directed U.S. diplomats to dress in the plain style of an ordinary American rather than the court-dress many had adopted from Europe. He also negotiated a reciprocity treaty with British North America and the Gadsden purchase with Mexico, acquiring territory in present-day Arizona and New Mexico. He left office in 1857 and died shortly thereafter.