John Caldwell Calhoun (/kælˈhuːn/; March 18, 1782 – March 31, 1850) was an American statesman and political theorist who served as the seventh vice president of the United States from 1825 to 1832. Calhoun began his political career as a nationalist, modernizer and proponent of a strong federal government and protective tariffs. In the late 1820s, his views shifted, and he became a leading proponent of states' rights, limited government, nullification, and opposition to high tariffs, and distinguished himself as an outspoken defender of American slavery. Calhoun saw Northern acceptance of those policies as a condition of the South remaining in the Union. His beliefs heavily influenced the South's secession from the Union in 1860 and 1861. Calhoun was the first of two vice presidents to resign from the position, the second being Spiro Agnew, who resigned in 1973.

Born in South Carolina, Calhoun began his political career with election to the House of Representatives in 1810. As a prominent leader of the war hawk faction, he strongly supported the War of 1812. Calhoun served as Secretary of War under President James Monroe and, in that position, reorganized and modernized the War Department. He was a candidate for the presidency in the 1824 election. After failing to gain support, Calhoun agreed to be a candidate for vice president. The Electoral College elected him vice president by an overwhelming majority. He served under John Quincy Adams and continued under Andrew Jackson, who defeated Adams in the election of 1828, making Calhoun the most recent U.S. vice president to serve under two different presidents.

Calhoun had a difficult relationship with Jackson, primarily because of the Nullification Crisis and the Petticoat affair. In contrast with his previous nationalist sentiments, Calhoun vigorously supported South Carolina's right to nullify federal tariff legislation that he believed unfairly favored the North, which put him into conflict with Unionists such as Jackson. In 1832, with only a few months remaining in his second term, Calhoun resigned as vice president and was elected to the Senate. He sought the Democratic Party nomination for the presidency in 1844 but lost to surprise nominee James K. Polk, who won the general election. Calhoun served as Secretary of State under President John Tyler from 1844 to 1845, and in that role supported the annexation of Texas as a means to extend the Slave Power and helped to settle the Oregon boundary dispute with Britain. Calhoun returned to the Senate, where he opposed the Mexican–American War, the Wilmot Proviso and the Compromise of 1850 before he died of tuberculosis in 1850. He often served as a virtual independent who variously aligned as needed with Democrats and Whigs.

Later in life, Calhoun became known as the "cast-iron man" for his rigid defense of white Southern beliefs and practices. His concept of republicanism emphasized proslavery thought and minority states' rights as embodied by the South. He owned dozens of slaves in Fort Hill, South Carolina, and asserted that slavery, rather than being a "necessary evil", was a "positive good" that benefited both slaves and enslavers. To protect minority rights against majority rule, he called for a concurrent majority by which the minority could block some proposals that it felt infringed on their liberties. To that end, Calhoun supported states' rights, and nullification, through which states could declare null and void federal laws that they viewed as unconstitutional. He was one of the "Great Triumvirate" or the "Immortal Trio" of congressional leaders, along with his colleagues Daniel Webster and Henry Clay.

John Caldwell Calhoun was born in Abbeville District, South Carolina on March 18, 1782. He was the fourth child of Irish-born Patrick Calhoun and his wife Martha Caldwell. Patrick's father, also named Patrick, joined the waves of Scotch-Irish emigration from County Donegal to southwestern Pennsylvania. After the death of the elder Patrick in 1741, the family moved to Virginia. Following the British defeat at the Battle of the Monongahela in 1755, the family, fearing Indian attacks, moved to South Carolina in 1756. Patrick, a prominent member of the tight-knit Scotch-Irish community on the frontier who worked as surveyor and farmer, was elected to the South Carolina Legislature in 1763 and acquired ownership over slave plantations. As a Presbyterian, he stood opposed to the established Anglican planter class based in Charleston. Patrick remained neutral in the American Revolution and opposed ratification of the U.S. Constitution on grounds of states' rights and personal liberties. Calhoun would eventually adopt his father's beliefs on states' rights.

Young Calhoun showed scholastic talent, and although schools were scarce on the Carolina frontier, he was enrolled briefly in an academy taught by his brother-in-law Moses Waddel. It stressed the Latin and Greek classics. He continued his studies privately. When his father died, his brothers were away starting business careers, and so the 14-year-old Calhoun took over management of the family farm and five other farms. For four years he simultaneously kept up his reading and his hunting and fishing. The family decided he should continue his education, and so he resumed studies at Waddel's academy after it reopened. With financing from his brothers, he went to Yale College in Connecticut in 1802. For the first time in his life, Calhoun encountered serious, advanced, and well-organized intellectual dialogue that could shape his mind. Yale was dominated by President Timothy Dwight, a Federalist who became his mentor. Dwight's brilliance entranced (and sometimes repelled) Calhoun.

Biographer John Niven says:

Calhoun admired Dwight's extemporaneous sermons, his seemingly encyclopedic knowledge, and his awesome mastery of the classics, of the tenets of Calvinism, and of metaphysics. No one, he thought, could explicate the language of John Locke with such clarity.