James Madison Jr. (March 16, 1751 – June 28, 1836) was an American statesman, diplomat, expansionist, philosopher, and Founding Father who served as the 4th president of the United States from 1809 to 1817. He 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. He co-wrote The Federalist Papers, co-founded the Democratic-Republican Party, and served as the 5th Secretary of State from 1801 to 1809 under President Thomas Jefferson.

Born into a prominent Virginia planter family, Madison served as a member of the Virginia House of Delegates and the Continental Congress during and after the American Revolutionary War. He became dissatisfied with the weak national government established by the Articles of Confederation and helped organize the Constitutional Convention, which produced a new constitution to supplant the Articles of Confederation. Madison's Virginia Plan served as the basis for the Constitutional Convention's deliberations, and he was one of the most influential individuals at the convention. He became one of the leaders in the movement to ratify the Constitution, and he joined with Alexander Hamilton and John Jay in writing The Federalist Papers, a series of pro-ratification essays that was one of the most influential works of political science in American history.

After the ratification of the Constitution, Madison emerged as an important leader in the House of Representatives and served as a close adviser to President George Washington. He is considered the main force behind the ratification of the Bill of Rights, which enshrines guarantees of personal freedoms and rights within the Constitution. During the early 1790s, Madison opposed the economic program and the accompanying centralization of power favored by Secretary of the Treasury Hamilton. Along with Jefferson, he organized the Democratic–Republican Party, which was, alongside Hamilton's Federalist Party, one of the nation's first major political parties. After Jefferson was elected president, Madison served as his Secretary of State from 1801 to 1809. In that position, he supervised the Louisiana Purchase, which doubled the size of the United States.

Madison succeeded Jefferson after his victory in the 1808 presidential election. After diplomatic protests and a trade embargo failed to end British seizures of American shipping, he led the United States into the War of 1812. The war was an administrative morass and ended inconclusively, but many Americans saw it as a successful "second war of independence" against Britain. As the war progressed, Madison was re-elected in 1812, albeit by a smaller margin to the 1808 election. 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 war, Madison's presidency added 23 million acres of Native American land to the United States.

Madison retired from public office after concluding his presidency in 1817 and died in 1836. Like Jefferson and Washington, Madison was a wealthy slave owner who never treated his slaves harshly and provided for them, but his slaves experienced "the lack of basic freedom of action and opportunity for advancement that defined the deep and profound discrepancy between enslaved and most free Virginians." He never privately reconciled his republican beliefs with his slave ownership. Forced to pay debts, he never freed his slaves. Madison is considered one of the most important Founding Fathers of the United States, and historians have generally ranked him as an above-average president.