James Madison Jr. (March 16, 1751 – June 28, 1836) was an American statesman, diplomat, and Founding Father. He served as the fourth president of the United States from 1809 to 1817. Madison is hailed as the "Father of the Constitution" for his pivotal role in drafting and promoting the Constitution of the United States and the Bill of Rights.

Madison was born into a prominent slave-owning planter family in Virginia. He served as a member of the Virginia House of Delegates and the Continental Congress during and after the American Revolutionary War. Unsatisfied with the weak national government established by the Articles of Confederation, he helped organize the Constitutional Convention, which produced a new constitution designed to strengthen republican government against democratic assembly. Madison's Virginia Plan was the basis for the Convention's deliberations, and he was an influential voice at the convention. He became one of the leaders in the movement to ratify the Constitution, and joined Alexander Hamilton and John Jay in writing The Federalist Papers, a series of pro-ratification essays which remains prominent among works of political science in American history. Madison emerged as an important leader in the House of Representatives and was a close adviser to President George Washington.

During the early 1790s, Madison opposed the economic program and the accompanying centralization of power favored by Secretary of the Treasury Hamilton. Alongside Thomas Jefferson, he organized the Democratic–Republican Party in opposition to Hamilton's Federalist Party. After Jefferson was elected president in 1800, Madison served as his Secretary of State from 1801 to 1809 and supported Jefferson in the case of Marbury v. Madison. While Madison was Secretary of State, Jefferson made the Louisiana Purchase, and later, as President, Madison oversaw related disputes in the Northwest territories.

Madison was elected president in 1808. Motivated by the desire for acquiring land held by Britain, Spain, and Native Americans, and after diplomatic protests with a trade embargo failed to end British seizures of American shipped goods, he led the United States into the War of 1812. Although the war ended inconclusively, many Americans viewed the war's outcome as a successful "second war of independence" against Britain. Madison was re-elected in 1812, albeit by a smaller margin. The war convinced Madison of the necessity of a stronger federal government. He presided over the creation of the Second Bank of the United States and the enactment of the protective Tariff of 1816. By treaty or through war, Native American tribes ceded 26,000,000 acres (11,000,000 ha) of land to the United States under Madison's presidency.

Retiring from public office at the end of his presidency in 1817, Madison returned to his plantation, Montpelier, and died there in 1836. During his lifetime, Madison was a slave owner. In 1783, at the height of Revolutionary politics, Madison freed one of his slaves. However, Madison did not free any slaves in his will. Among historians, Madison is considered one of the most important Founding Fathers of the United States. Leading historians have generally ranked him as an above-average president, although they are critical of his endorsement of slavery and his leadership during the War of 1812. Madison's name is commemorated in many landmarks across the nation, both publicly and privately, with prominent examples including Madison Square Garden, James Madison University, and the USS James Madison.