James Knox Polk (/poʊk/; November 2, 1795 – June 15, 1849) was the 11th president of the United States, serving from 1845 to 1849. A protégé of Andrew Jackson and a member of the Democratic Party, he was an advocate of American expansionism and Jacksonian democracy. Polk saw Texas join the Union in his first year in office, one of the precipitating causes that soon led the U.S. into the Mexican–American War. The settlement of that war expanded American territory to the Pacific Ocean. During his term, the dispute over the Oregon Territory with the United Kingdom was resolved as well, creating the present U.S.-Canadian boundary.

After building a successful law practice in Tennessee, Polk was elected to its state legislature in 1823 and then to the United States House of Representatives in 1825, becoming a strong supporter of Jackson. Following his tenure as chairman of the Ways and Means Committee, he became Speaker of the House in 1835, the only person to serve both as Speaker and U.S. president. Polk left Congress to run for governor of Tennessee, winning in 1839 but losing in 1841 and 1843. He was a dark-horse candidate in the 1844 presidential election as the Democratic Party nominee. Polk entered his party's convention as a potential nominee for vice president but emerged as a compromise to head the ticket when no presidential candidate could gain the necessary two-thirds majority. In the general election, he narrowly defeated Henry Clay of the Whig Party and pledged to serve only one term.

After a negotiation fraught with the risk of war, Polk reached a settlement with Great Britain over the disputed Oregon Country, with the territory for the most part divided along the 49th parallel. He oversaw victory in the Mexican–American War, resulting in Mexico's cession of the entire American Southwest. He secured a substantial reduction of tariff rates with the Walker tariff of 1846. The same year, he achieved his other major goal, reestablishment of the Independent Treasury system. True to his campaign pledge to serve one term (one of the few U.S. presidents to make and keep such a pledge), Polk left office in 1849 and returned to Tennessee, where he died of cholera soon afterward.

Though he has become relatively obscure, scholars have ranked Polk mostly in the upper tier of U.S. presidents, mostly for his ability to promote and achieve the major items on his presidential agenda. At the same time, he has been criticized for leading the country into a war with Mexico that exacerbated sectional divides. A property owner who used slave labor, he kept a plantation in Mississippi and increased his slave ownership during his presidency. Polk's policy of territorial expansion saw the nation reach the Pacific coast and almost all its contiguous borders. He helped make the U.S. a nation poised to become a world power, but with divisions between free and slave states gravely exacerbated, setting the stage for the Civil War.

James Knox Polk was born on November 2, 1795, in a log cabin in Pineville, North Carolina. He was the first of 10 children born into a family of farmers. His mother Jane named him after her father, James Knox. His father Samuel Polk was a farmer, slaveholder, and surveyor of Scots-Irish descent. The Polks had immigrated to America in the late 17th century, settling initially on the Eastern Shore of Maryland but later moving to south-central Pennsylvania and then to the Carolina hill country.

The Knox and Polk families were Presbyterian. While Polk's mother remained a devout Presbyterian, his father, whose own father Ezekiel Polk was a deist, rejected dogmatic Presbyterianism. He refused to declare his belief in Christianity at his son's baptism, and the minister refused to baptize young James. Nevertheless, James' mother "stamped her rigid orthodoxy on James, instilling lifelong Calvinistic traits of self-discipline, hard work, piety, individualism, and a belief in the imperfection of human nature", according to James A. Rawley's American National Biography article.

In 1803, Ezekiel Polk led four of his adult children and their families to the Duck River area in what became Maury County, Tennessee; Samuel Polk and his family followed in 1806. The Polk clan dominated politics in Maury County and in the new town of Columbia. Samuel became a county judge, and the guests at his home included Andrew Jackson, who had already served as a judge and in Congress. James learned from the political talk around the dinner table; both Samuel and Ezekiel were strong supporters of President Thomas Jefferson and opponents of the Federalist Party.

Polk suffered from poor health as a child, a particular disadvantage in a frontier society. His father took him to see prominent Philadelphia physician Dr. Philip Syng Physick for urinary stones. The journey was broken off by James's severe pain, and Dr. Ephraim McDowell of Danville, Kentucky, operated to remove them. No anesthetic was available at that time except brandy. The operation was successful, but it may have left James impotent or sterile, as he had no children. He recovered quickly and became more robust. His father offered to bring him into one of his businesses, but he wanted an education and enrolled at a Presbyterian academy in 1813. He became a member of the Zion Church near his home in 1813 and enrolled in the Zion Church Academy. He then entered Bradley Academy in Murfreesboro, Tennessee, where he proved a promising student.