Thomas Jefferson (April 13, 1743 – July 4, 1826) was the third President of the United States (1801–1809), the principal author of the Declaration of Independence (1776), and—for his promotion of the ideals of republicanism in the United States—one of the most influential Founding Fathers. Jefferson envisioned America as the force behind a great "Empire of Liberty" that would promote republicanism and counter the imperialism of the British Empire.

Major events during his presidency include the Louisiana Purchase (1803) and the Lewis and Clark Expedition (1804–1806), as well as escalating tensions with both Britain and France that led to war with Britain in 1812, after he left office.

As a political philosopher, Jefferson was a man of the Enlightenment and knew many intellectual leaders in Britain and France. He idealized the independent yeoman farmer as exemplar of republican virtues, distrusted cities and financiers, and favored states' rights and a strictly limited federal government. Jefferson supported the separation of church and state and was the author of the Virginia Statute for Religious Freedom (1779, 1786). He was the eponym of Jeffersonian democracy and the cofounder and leader of the Democratic-Republican Party, which dominated American politics for 25 years. Jefferson served as the wartime Governor of Virginia (1779–1781), first United States Secretary of State (1789–1793), and second Vice President (1797–1801).

A polymath, Jefferson achieved distinction as, among other things, a horticulturist, political leader, architect, archaeologist, paleontologist, inventor, and founder of the University of Virginia. When President John F. Kennedy welcomed 49 Nobel Prize winners to the White House in 1962 he said, "I think this is the most extraordinary collection of talent and of human knowledge that has ever been gathered together at the White House – with the possible exception of when Thomas Jefferson dined alone." To date, Jefferson is the only president to serve two full terms in office without vetoing a single bill of Congress. Jefferson has been consistently ranked by scholars as one of the greatest of U.S. presidents.

Thomas Jefferson was born on April 13, 1743 into a family closely related to some of the most prominent individuals in Virginia, the third of ten children. Two died in childhood. His mother was Jane Randolph, daughter of Isham Randolph, a ship's captain and sometime planter. Jefferson's father was Peter Jefferson, a planter and surveyor in Albemarle County (Shadwell, then Edge Hill, Virginia.) He was of Welsh descent. When Colonel William Randolph, an old friend of Peter Jefferson, died in 1745, Peter assumed executorship and personal charge of William Randolph's estate in Tuckahoe as well as his infant son, Thomas Mann Randolph, Jr. That year the Jeffersons relocated to Tuckahoe where they would remain for the next seven years before returning to their home in Albemarle. Peter Jefferson was then appointed to the Colonelcy of the county, an important position at the time. In 1752, Jefferson began attending a local school run by William Douglas, a Scottish minister. At the age of nine, Jefferson began studying Latin, Greek, and French. In 1757, when he was 14 years old, his father died. Jefferson inherited about 5,000 acres (20 km²) of land and dozens of slaves. He built his home there, which eventually became known as Monticello. After his father's death, he was taught at the school of the learned minister James Maury from 1758 to 1760. The school was in Fredericksville Parish near Gordonsville, Virginia, twelve miles (19 km) from Shadwell, and Jefferson boarded with Maury's family. There he received a classical education and studied history and science.

In 1760 Jefferson entered the College of William & Mary in Williamsburg at the age of 16; he studied there for two years, graduating with highest honors in 1762. At William & Mary, he enrolled in the philosophy school and studied mathematics, metaphysics, and philosophy under Professor William Small, who introduced the enthusiastic Jefferson to the writings of the British Empiricists, including John Locke, Francis Bacon, and Isaac Newton (Jefferson called them the "three greatest men the world had ever produced"). He also perfected his French, carried his Greek grammar book wherever he went, practiced the violin, and read Tacitus and Homer. A keen and diligent student, Jefferson displayed an avid curiosity in all fields and, according to the family tradition, frequently studied fifteen hours a day. His closest college friend, John Page of Rosewell, reported that Jefferson "could tear himself away from his dearest friends to fly to his studies." While in college, Jefferson was a member of a secret organization called the F.H.C. Society. He lodged and boarded at the College in the building known today as the Sir Christopher Wren Building, attending communal meals in the Great Hall, and morning and evening prayers in the Wren Chapel. Jefferson often attended the lavish parties of royal governor Francis Fauquier, where he played his violin and developed an early love for wines. After graduating in 1762 with highest honors, he read law with George Wythe and was admitted to the Virginia bar in 1767.

On October 1, 1765, Jefferson's oldest sister Jane died at the age of 25. Jefferson fell into a period of deep mourning, as he was already saddened by the absence of his sisters Mary, who had been married several years to Thomas Bolling, and Martha, who had wed earlier in July to Dabney Carr. Both had moved to their husbands' residences, leaving younger siblings Elizabeth, Lucy, and the two toddlers as his companions. Jefferson was not comforted by the presence of Elizabeth or Lucy as they did not provide him with the same intellectual stimulation as his older siblings had.

Jefferson would go on to handle many cases as a lawyer in colonial Virginia, managing more than a hundred cases each year between 1768 and 1773 in General Court alone, while acting as counsel in hundreds of cases. Jefferson's client list included members of the Virginia's elite families, including members of his mother's family, the Randolphs. In 1768 Thomas Jefferson started the construction of Monticello, a neoclassical mansion. Starting in childhood, Jefferson had always wanted to build a beautiful mountaintop home within site of Shadwell. Jefferson went greatly in debt on Monticello by spending lavishly on his Monticello Estate to create a neoclassical environment, based on his study of the brilliant architect Andrea Palladio and The Orders. Monticello was also Thomas Jefferson's slave plantation. Throughout a period lasting seventy years, Thomas Jefferson owned over 600 slaves. Many of the slaves at the Monticello plantation intermarried amongst each other and produced children. Jefferson only paid a few of his trusted slaves in important positions for work done or for performing difficult tasks like cleaning chimneys or privies. Although there are no direct workday references, Jefferson’s slaves probably worked from dawn to dusk, with shorter or longer days according to the season. Fragmentary records indicate a rich spiritual life at Monticello slave quarters, incorporating both Christian and African traditions. Although there is no record that Jefferson instructed slaves in grammar education, several enslaved men at Monticello could read and write.

In 1772, at age 29 Jefferson married the 23-year-old widow Martha Wayles Skelton. They had six children: Martha Jefferson Randolph (1772–1836), Jane Randolph (1774–1775), a stillborn or unnamed son (1777), Mary Jefferson Eppes (1778–1804), Lucy Elizabeth (1780–1781), and another Lucy Elizabeth (1782–1785). Martha died on September 6, 1782, after the birth of her last child. Jefferson never remarried. Jefferson is alleged to have had a long-term, intimate relationship with one of his slaves, Sally Hemings, a quadroon, who was believed to have been a half-sister to Jefferson's late wife. During the administration of President Jefferson, journalists and others alleged that he had fathered several children with Hemings after his wife's death. The allegation that Jefferson fathered children with Hemings had been a topic of local gossip for years before, when controversial journalist James T. Callender published in the September 1, 1802, issue of the Richmond Recorder, a Virginia newspaper, ... "[Jefferson] keeps and for many years has kept, as his concubine, one of his slaves. Her name is Sally." Jefferson never responded publicly about this issue, but was said to have denied having a carnal relationship with Hemings in his private correspondence.

Besides practicing law, Jefferson represented Albemarle County in the Virginia House of Burgesses beginning in 1769. Following the passage of the Coercive Acts by the British Parliament in 1774, he wrote a set of resolutions against the acts, which were expanded into A Summary View of the Rights of British America, his first published work. Previous criticism of the Coercive Acts had focused on legal and constitutional issues, but Jefferson offered the radical notion that the colonists had the natural right to govern themselves. Jefferson also argued that Parliament was the legislature of Great Britain only, and had no legislative authority in the colonies. The paper was intended to serve as instructions for the Virginia delegation of the First Continental Congress, but Jefferson's ideas proved to be too radical for that body. Nevertheless, the pamphlet helped provide the theoretical framework for American independence, and marked Jefferson as one of the most thoughtful patriot spokesmen.

Jefferson served as a delegate to the Second Continental Congress beginning in June 1775, soon after the outbreak of the American Revolutionary War. When Congress began considering a resolution of independence in June 1776, Jefferson was appointed to a five-man committee to prepare a declaration to accompany the resolution. The committee selected Jefferson to write the first draft probably because of his reputation as a writer. The assignment was considered routine; no one at the time thought that it was a major responsibility. Jefferson completed a draft in consultation with other committee members, drawing on his own proposed draft of the Virginia Constitution, George Mason's draft of the Virginia Declaration of Rights, and other sources. Jefferson showed his draft to the committee, which made some final revisions, and then presented it to Congress on June 28, 1776. After voting in favor of the resolution of independence on July 2, Congress turned its attention to the declaration. Over several days of debate, Congress made a few changes in wording and deleted nearly a fourth of the text, most notably a passage critical of the slave trade, changes that Jefferson resented. On July 4, 1776, the wording of the Declaration of Independence was approved. The Declaration would eventually become Jefferson's major claim to fame, and his eloquent preamble became an enduring statement of human rights.

In September 1776, Jefferson returned to Virginia and was elected to the new Virginia House of Delegates. During his term in the House, Jefferson set out to reform and update Virginia's system of laws to reflect its new status as a democratic state. He drafted 126 bills in three years, including laws to abolish primogeniture, establish freedom of religion, and streamline the judicial system. In 1778, Jefferson's "Bill for the More General Diffusion of Knowledge" led to several academic reforms at his alma mater, including an elective system of study—the first in an American university. While in the state legislature Jefferson proposed a bill to eliminate capital punishment for all crimes except murder and treason. His effort to reform the death penalty law was defeated by just one vote, and such crimes as rape remained punishable by death in Virginia until the 1960s. He succeeded in passing an act prohibiting the importation of slaves but not slavery itself. Jefferson served as governor of Virginia from 1779–1781. As governor, he oversaw the transfer of the state capital from Williamsburg to the more central location of Richmond in 1780. He continued to advocate educational reforms at the College of William and Mary, including the nation's first student-policed honor code. In 1779, at Jefferson's behest, William and Mary appointed George Wythe to be the first professor of law in an American university. Dissatisfied with the rate of changes he wanted to push through, he later became the founder of the University of Virginia, which was the first university in the United States at which higher education was completely separate from religious doctrine.

Virginia was invaded twice by the British led first by Benedict Arnold and then by Lord Cornwallis during Jefferson's term as governor. He, along with Patrick Henry and other leaders of Virginia, were but ten minutes away from being captured by Banastre Tarleton, a British colonel leading a cavalry column that was raiding the area in June 1781. Public disapproval of his performance delayed his future political prospects, and he was never again elected to office in Virginia. He was, however, appointed by the state legislature to Congress in 1783. The Virginia state legislature appointed Jefferson to the Congress of the Confederation on 6 June 1783, his term beginning on 1 November. He was a member of the committee set up to set foreign exchange rates, and in that capacity he recommended that the American currency should be based on the decimal system. Jefferson also recommended setting up the Committee of the States, to function as the executive arm of Congress when Congress was in session.

He left Congress when he was elected a minister plenipotentiary on 7 May 1784. He became Minister to France in 1785. Because Jefferson served as minister to France from 1785 to 1789, he was not able to attend the Philadelphia Convention. He generally supported the new constitution despite the lack of a bill of rights and was kept informed by his correspondence with James Madison. While in Paris, he lived in a home on the Champs-Élysées. He spent much of his time exploring the architectural sites of the city, as well as enjoying the fine arts that Paris had to offer. He became a favorite in the salon culture and was a frequent dinner guest of many of the city's most prominent people. In addition, he frequently entertained others from French and European society. He and his daughters were accompanied by two slaves of the Hemings family from Monticello. Jefferson paid for James Hemings to be trained as a French chef (Hemings later accompanied Jefferson as chef when he was in Philadelphia). Sally Hemings, James' sister, had accompanied Jefferson's younger daughter overseas. Jefferson is believed to have begun his long-term relationship with Sally Hemings in Paris. Both the Hemings learned French during their time in the city. From 1784 to 1785, Jefferson was one of the architects of trade relations between the United States and Prussia. The Prussian ambassador Friedrich Wilhelm von Thulemeyer and John Adams, both living in the Hague, and Benjamin Franklin in Paris, were also involved. Despite his numerous friendships with the social and noble elite, when the French Revolution began in 1789, Jefferson sided with the revolutionaries.

After returning from France, Jefferson served as the first Secretary of State under George Washington (1790–1793). Jefferson and Alexander Hamilton began sparring over national fiscal policy, especially the funding of the debts of the war, with Hamilton believing that the debts should be equally shared, and Jefferson believing that each state should be responsible for its own debt (Virginia had not accumulated much debt during the Revolution). In further sparring with the Federalists, Jefferson came to equate Hamilton and the rest of the Federalists with Tories and monarchists who threatened to undermine republicanism. He equated Federalism with "Royalism," and made a point to state that "Hamiltonians were panting after...and itching for crowns, coronets and mitres." Jefferson and James Madison founded and led the Democratic-Republican Party. He worked with Madison and his campaign manager John J. Beckley to build a nationwide network of Republican allies to combat Federalists across the country.

Jefferson strongly supported France against Britain when war broke out between those nations in 1793. Historian Lawrence S. Kaplan notes Jefferson's "visceral support for the French cause," while agreeing with Washington that the nation should not get involved in the fighting. The arrival in 1793 of an aggressive new French minister, Edmond-Charles Genêt, caused a crisis for the Secretary of State, as he watched Genêt try to violate American neutrality, manipulate public opinion, and even go over Washington's head in appealing to the people; projects that Jefferson helped to thwart. Jefferson still clung to his sympathies with France and hoped for the success of her arms abroad and a cordial compact with her at home. Jefferson at the end of 1793 retired to Monticello where he continued to orchestrate opposition to Hamilton and Washington. However, the Jay Treaty of 1794, orchestrated by Hamilton, brought peace and trade with Britain – while Madison, with strong support from Jefferson, wanted "to strangle the former mother country" without going to war. "It became an article of faith among Republicans that 'commercial weapons' would suffice to bring Great Britain to any terms the United States chose to dictate." Jefferson, in retirement, strongly encouraged Madison. As the Democratic-Republican candidate in 1796 he lost to John Adams, but had enough electoral votes to become Vice President (1797–1801). He wrote a manual of parliamentary procedure, but otherwise avoided the Senate.

With the Quasi-War, an undeclared naval war with France, underway, the Federalists under John Adams started a navy, built up the army, levied new taxes, readied for war, and enacted the Alien and Sedition Acts in 1798. Jefferson interpreted the Alien and Sedition Acts as an attack on his party more than on dangerous enemy aliens; they were used to attack his party, with the most notable attacks coming from Matthew Lyon, a representative from Vermont. Jefferson and Madison rallied support by anonymously writing the Kentucky and Virginia Resolutions, which declared that the federal government had no right to exercise powers not specifically delegated to it by the states. The Resolutions meant that, should the federal government assume such powers, its acts under them could be voided by a state. The Resolutions presented the first statements of the states' rights theory, that later led to the concepts of nullification and interposition. Working closely with Aaron Burr of New York, Jefferson rallied his party, attacking the new taxes especially, and ran for the Presidency in 1800. Consistent with the traditions of the times, he did not formally campaign for the position. Before the passage of the Twelfth Amendment, a problem with the new union's electoral system arose. He tied with Burr for first place in the electoral college, leaving the House of Representatives (where the Federalists still had some power) to decide the election. After lengthy debate within the Federalist-controlled House, Hamilton convinced his party that Jefferson would be a lesser political evil than Burr and that such scandal within the electoral process would undermine the still-young regime. The issue was resolved by the House, on February 17, 1801, after thirty-six ballots, when Jefferson was elected President and Burr Vice President. Burr's refusal to remove himself from consideration created ill will with Jefferson, who dropped Burr from the ticket in 1804 after Burr killed Hamilton in a duel.

However, Jefferson's win over the Federalist John Adams in the general election was derided in its time for how the electoral college was set up under the three-fifths compromise at the Constitutional convention. Jefferson owed part of his election to the South's inflated number of Electors due to slave-holdings, which meant that twelve of Jefferson's electoral votes—his margin of victory—were derived from citizenry who were denied the vote and their full humanity. After his election in 1800, Jefferson was derided as the "Negro President", with critics like the Mercury and New-England Palladium of Boston writing on January 20, 1801, that Jefferson had the gall to celebrate his election as a victory for democracy when he won "the temple of Liberty on the shoulders of slaves."

Jefferson repealed many federal taxes, and sought to rely mainly on customs revenue. He pardoned people who had been imprisoned under the Alien and Sedition Acts, passed in John Adams' term, which Jefferson believed to be unconstitutional. He repealed the Judiciary Act of 1801 and removed many of Adams' "midnight judges" from office, which led to the Supreme Court deciding the important case of Marbury v. Madison. He began and won the First Barbary War (1801–1805), America's first significant overseas war, and established the United States Military Academy at West Point in 1802. In 1803, despite his misgivings about the constitutionality of Congress's power to buy land, Jefferson bought Louisiana from France, doubling the size of the United States. The land thus acquired amounts to 23 percent of the United States today. In 1807 his former vice president, Aaron Burr, was tried for treason on Jefferson's order, but was acquitted. During the trial Chief Justice John Marshall subpoenaed Jefferson, who invoked executive privilege and claimed that as president he did not need to comply. When Marshall held that the Constitution did not provide the president with any exception to the duty to obey a court order, Jefferson backed down. Jefferson's reputation was damaged by the Embargo Act of 1807, which was ineffective and was repealed at the end of his second term. In 1803, President Jefferson signed into law a bill that excluded blacks from carrying the U.S. mail. Historian John Hope Franklin called the signing "a gratuitous expression of distrust of free Negroes who had done nothing to merit it." On March 3, 1807, Jefferson signed a bill making slave importation illegal in the United States.

After leaving the Presidency, Jefferson continued to be active in public affairs. He also became increasingly concerned with founding a new institution of higher learning, specifically one free of church influences where students could specialize in many new areas not offered at other universities. Jefferson believed educating people was a good way to establish an organized society, and felt schools should be paid for by the general public, so less wealthy people could obtain student membership as well. A letter to Joseph Priestley, in January 1800, indicated that he had been planning the University for decades before its establishment. His dream was realized in 1819 with the founding of the University of Virginia. Upon its opening in 1825, it was then the first university to offer a full slate of elective courses to its students. One of the largest construction projects to that time in North America, it was notable for being centered about a library rather than a church. No campus chapel was included in his original plans. Until his death, Jefferson invited students and faculty of the school to his home. Jefferson is widely recognized for his architectural planning of the University of Virginia grounds, an innovative design that is a powerful representation of his aspirations for both state sponsored education and an agrarian democracy in the new Republic. His educational idea of creating specialized units of learning is physically expressed in the configuration of his campus plan, which he called the "Academical Village." Individual academic units are expressed visually as distinct structures, represented by Pavilions, facing a grassy quadrangle, with each Pavilion housing classroom, faculty office, and homes. His highly ordered site plan establishes an ensemble of buildings surrounding a central rectangular quadrangle, named The Lawn, which is lined on either side with the academic teaching units and their linking arcades. The quad is enclosed at one end with the library, the repository of knowledge, at the head of the table. The remaining side opposite the library remained open-ended for future growth. The lawn rises gradually as a series of stepped terraces, each a few feet higher than the last, rising up to the library set in the most prominent position at the top, while also suggesting that the Academical Village facilitates easier movement to the future.
Stylistically, Jefferson was a proponent of the Greek and Roman styles, which he believed to be most representative of American democracy by historical association. Each academic unit is designed with a two story temple front facing the quadrangle, while the library is modeled on the Roman Pantheon. The ensemble of buildings surrounding the quad is an unmistakable architectural statement of the importance of secular public education, while the exclusion of religious structures reinforces the principle of separation of church and state. The campus planning and architectural treatment remains today as a paradigm of building of structures to express intellectual ideas and aspirations. A survey of members of the American Institute of Architects identified Jefferson's campus as the most significant work of architecture in America. The University was designed as the capstone of the educational system of Virginia. In his vision, any citizen of the state could attend school with the sole criterion being ability.

Jefferson died on July 4, 1826, the fiftieth anniversary of the adoption of the Declaration of Independence. He died a few hours before John Adams, his compatriot in their quest for independence, then great political rival, and later friend and correspondent. Adams is often rumored to have referenced Jefferson in his last words, unaware of his passing. Although he was born into one of the wealthiest families in North America, Thomas Jefferson was deeply in debt when he died. After his death, his possessions were sold at auction.Thomas Jefferson is buried on his Monticello estate, in  Charlottesville, Virginia. In his will, he left Monticello to the United States to be used as a school for orphans of navy officers. His epitaph, written by him with an insistence that only his words and "not a word more" be inscribed (notably omitting his service as Governor of Virginia, Vice-President and President), reads:

Below the epitaph on a separate panel is written:

The initials O.S. are a notation for Old Style and that is a reference to the change of dating that occurred during Jefferson's lifetime from the Julian calendar to the Gregorian calendar under the British Calendar (New Style) Act 1750.