William Learned Marcy (December 12, 1786–July 4, 1857) was an American statesman. He was born in Southbridge, Massachusetts, graduated from Brown University, taught school in Newport, Rhode Island, studied law; was admitted to the bar in 1811 and commenced practice in Troy, New York.

He served in the War of 1812; was recorder of Troy for several years, but as he sided with the Anti-Clinton faction of the Democratic-Republican Party, known as the Bucktails , he was removed from office in 1818 by his political opponents. He served as editor of the Troy Budget, was named adjutant-general of the New York militia in 1821, and was New York state comptroller from 1823 until 1829. During this period he became the leading member of the famous Albany Regency, a group of able Democratic politicians who controlled much of the state.

He was an associate justice of the New York State Supreme Court, was elected as a Jacksonian Democrat to the United States Senate and served from 1831 until 1833, including on the U.S. Senate Committee on the Judiciary in the 22nd Congress, when he became Governor of New York, a position he held from 1833 until 1839. (He was defeated in 1838 for re-election by the United States Whig Party candidate, William H. Seward.) He was a member of the Mexican Claims Commission 1839 to 1842. From 1844 to 1845 he was recognized as one of the leaders of the Hunkers, or regular Democrats in New York, and an active opponent of the Barnburners.

Served as United States Secretary of War in the Cabinet of President James K. Polk from 1845 until 1849, at which time he resumed the practice of law. After 1849, Marcy led the "Soft" faction of the Hunkers that supported reconciliation with the Barnburners, and in this role sought the Democratic presidential nomination in 1852, but was unsuccessful, in part due to "Hard" opposition led by Daniel S. Dickinson. Marcy returned to public life in 1853 to serve as United States Secretary of State under President Franklin Pierce. According to the 1911 Encyclopædia Britannica, "His circular of the 1st of June 1853 to American diplomatic agents abroad, recommending that, whenever practicable, they should appear in the simple dress of an American citizen, created much discussion in Europe; in 1867 his recommendation was enacted into a law of Congress." He also resolved the Koszta Affair , and negotiated the Gadsden Purchase.

He died at Ballston Spa, New York is buried in the Rural Cemetery at Albany, New York. Mount Marcy, the highest peak in New York, is named after him.

|- style="text-align: center;" | width="30%" |Preceded by:
William Wilkins | width="40%" style="text-align: center;" |United States Secretary of War
1845–1849 | width="30%" |Succeeded by:
George Walker Crawford

|- style="text-align: center;" | width="30%" |Preceded by:
Edward Everett | width="40%" style="text-align: center;" |United States Secretary of State
March 7, 1853 – March 6, 1857 | width="30%" |Succeeded by:
Lewis Cass