Thomas Jefferson (April 13, 1743 – July 4, 1826) was an American statesman, diplomat, lawyer, architect, philosopher, and Founding Father who served as the third president of the United States from 1801 to 1809. Among the Committee of Five charged by the Second Continental Congress with authoring the Declaration of Independence, Jefferson was the primary author. Following the American Revolutionary War and prior to becoming president in 1801, Jefferson was the first United States secretary of state under George Washington and then the nation's second vice president under John Adams.

Jefferson's writings and advocacy for human rights, including freedom of thought, speech, and religion, were a leading inspiration behind the American Revolution, which ultimately led to American independence and the United States Constitution. He was a leading proponent of democracy, republicanism, and individual rights, and produced formative documents and decisions at the state, national, and international levels.

During the American Revolution, Jefferson represented Virginia in the Second Continental Congress in Philadelphia, which adopted the Declaration of Independence on July 4, 1776, and served as the second Governor of Virginia from 1779 to 1781. In 1785, Jefferson was appointed the United States Minister to France and subsequently the nation's first secretary of state under George Washington from 1790 to 1793. Jefferson and James Madison organized the Democratic-Republican Party to oppose the Federalist Party during the formation of the First Party System. With Madison, he anonymously wrote the Kentucky and Virginia Resolutions in 1798 and 1799, which sought to strengthen states' rights by nullifying the federal Alien and Sedition Acts.

Jefferson and Federalist John Adams became friends as well as political rivals, serving in the Continental Congress and drafting the Declaration of Independence together. In the 1796 presidential election between the two, Jefferson came in second, which according to electoral procedure at the time, made him Adams' vice president. Jefferson challenged Adams again in 1800 and won the presidency. After his term in office, Jefferson eventually reconciled with Adams and they shared a correspondence that lasted 14 years. He and Adams both died on the same day, July 4, 1826, the 50th anniversary of Declaration of Independence.

As president, Jefferson pursued the nation's shipping and trade interests against Barbary pirates and aggressive British trade policies. Beginning in 1803, he promoted a western expansionist policy with the Louisiana Purchase, which doubled the nation's claimed land area. To make room for settlement, Jefferson began the process of Indian tribal removal from the newly acquired territory. As a result of peace negotiations with France, his administration reduced military forces. He was re-elected in 1804, with his second term beset by difficulties at home, including the trial of former vice president Aaron Burr. In 1807, American foreign trade was diminished when Jefferson implemented the Embargo Act in response to British threats to U.S. shipping. The same year, Jefferson signed the Act Prohibiting Importation of Slaves.

Jefferson was a slave and plantation owner. Since the 1790s, he was rumored to have had children by his sister-in-law and slave Sally Hemings. According to scholarly consensus, Jefferson probably fathered at least six children with Hemings, including four that survived to adulthood. After retiring from public office, Jefferson founded the University of Virginia. Presidential scholars and historians generally praise Jefferson's public achievements, including his advocacy of religious freedom and tolerance, his peaceful acquisition of the Louisiana Territory from France, and the successful Lewis and Clark Expedition. Jefferson is consistently ranked among the top ten presidents in American history.