Thomas Francis Bayard (October 29, 1828 – September 28, 1898) was an American lawyer, politician and diplomat from Wilmington, Delaware. A Democrat, he served three terms as the United States Senator from Delaware and made three unsuccessful bids for the Democratic nomination for President of the United States. In 1885, President Grover Cleveland appointed him Secretary of State. After four years in private life, he returned to the diplomatic arena as Ambassador to Great Britain.

Born in Delaware to a prominent family, Bayard learned politics from his father James A. Bayard Jr., who also served in the Senate. In 1869, the Delaware legislature elected Bayard to the Senate upon his father's retirement. A Peace Democrat during the Civil War, Bayard spent his early years in the Senate in opposition to Republican policies, especially the Reconstruction of the defeated Confederate states. His conservatism extended to financial matters as he became known as a staunch supporter of the gold standard and an opponent of greenbacks and silver coinage which he believed would cause inflation. Bayard's conservative politics made him popular in the Southern United States and with financial interests in the Eastern United States, but never popular enough to obtain the Democratic nomination for president which he attempted to win in 1876, 1880 and 1884.

In 1885, President Cleveland appointed Bayard Secretary of State. Bayard worked with Cleveland to promote American trade in the Pacific while avoiding the acquisition of colonies at a time when many Americans clamored for them. He sought increased cooperation with the United Kingdom of Great Britain and Ireland, working to resolve disputes over fishing and seal-hunting rights in the waters around the Canada–United States border. As ambassador, Bayard continued to strive for Anglo-American friendship. This brought him into conflict with his successor at the State Department Richard Olney, when Olney and Cleveland demanded more aggressive diplomatic overtures than Bayard wished in the Venezuelan crisis of 1895. His term at the American embassy ended in 1897 and he died the following year.

Bayard was born in Wilmington, Delaware, in 1828, the second son of James A. Bayard Jr. and Anne née Francis. The Bayard family was prominent in Delaware, as Bayard's father would be elected to the United States Senate in 1851. Among Bayard's ancestors were his grandfather James A. Bayard, also a U.S. senator; and great-grandfather Richard Bassett, who served as U.S senator from and governor of Delaware. Several other relatives served in high office, including Bayard's uncle Richard H. Bayard, another Delaware senator; and his great-great-granduncle Nicholas Bayard, who was mayor of New York City. On his mother's side, Bayard descended from Philadelphia lawyer and financier Tench Francis Jr. Bayard was educated in private academies in Wilmington and then in Flushing, New York, when his father moved to New York City for business reasons. Bayard remained in New York when his father returned to Delaware in 1843, and he worked as a clerk in the mercantile firm of his brother-in-law August Schermerhorn.

In 1846, his father secured him a job in a banking firm in Philadelphia, and he worked there for the next two years. Bayard was unsatisfied with his progress at the firm and returned to Wilmington to read law at his father's office. Bayard was admitted to the bar in 1851, the year that his father was elected to the U.S. Senate. Thomas took on greater responsibilities in the family law office and rose quickly in the legal profession. In 1853, after the election of Democratic President Franklin Pierce, Bayard was appointed United States Attorney for Delaware. He spent only a year in the position before moving to Philadelphia to open a law practice with his friend William Shippen, a partnership that lasted until Shippen's death in 1858. While in Philadelphia, Bayard met Louise Lee, whom he married in October 1856. The marriage produced twelve children.

Bayard's return to Wilmington in 1858 brought greater involvement in the political scene. James Bayard was a delegate to the 1860 Democratic National Convention, and Thomas attended with him. The elder Bayard supported Robert M. T. Hunter of Virginia for the nomination. When the convention deadlocked and the Southern Democrats split from the main party, James Bayard adhered to the regular Democrats, but told Thomas that he thought the nominee, Stephen A. Douglas of Illinois, was untrustworthy. The subsequent election of Republican Abraham Lincoln and secession of the seven states of the Deep South led both Bayards to fear for the future of the Union, and the elder Bayard to propose a convention of all the states to resolve their differences. In the meantime, as four more Southern states seceded, James Bayard encouraged his son to help organize an independent militia unit, the Delaware Guard; Thomas Bayard was commissioned as its First Lieutenant.

In 1860, Delaware occupied an unusual position in the free state-slave state divide; nominally a slave state, Delaware's slave population had been in steep decline for decades and represented just 1.6% of the state's people. Opinion on secession was mixed in Delaware, but the Bayards were Peace Democrats and leaned to the Southern perspective. They blamed the war on abolitionist Republicans and believed that secession, while unwise, should not be suppressed with military force. Thomas Bayard spoke at a public meeting in Dover in June 1861, saying that "with this secession, or revolution, or rebellion, or by whatever name it may be called, the State of Delaware has naught to do." Even after the Civil War's first battles erupted in Virginia, Bayard continued to hope for peace. By early 1862, the Delaware Guard came under suspicion of Southern sympathies, and Major General Henry du Pont, commander of the state militia, ordered it disarmed. When Bayard refused to comply, he was briefly arrested before being released on parole. Bayard's father was reelected to the Senate in 1862, but resigned shortly thereafter in protest of the new oath of office, which demanded that Senators swear they had never borne arms against the United States nor given aid and encouragement to its enemies.

Bayard and his father continued in private law practice through the war. Both were pleased with the Democrats' peace platform in 1864, but disappointed in the choice of nominee, Major General George B. McClellan, a War Democrat. In 1866, Thomas Bayard successfully represented four South Carolinians in habeas corpus cases against the military. The following year, Senator George R. Riddle died and the legislature elected James Bayard to fill the remainder of the term, which ended in 1869. Bayard became more politically active, speaking at a public meeting in September 1867 against constitutional proposals for ending racial discrimination in voting rights. The following year, he condemned the impeachment proceedings against President Andrew Johnson, who had succeeded to the presidency in 1865 after Lincoln's assassination and had threatened the Republican Congress's plans for Reconstruction of the Southern states. Both Bayards attended the 1868 Democratic National Convention and, although they were unenthusiastic about the nominee, Horatio Seymour, supported the unsuccessful ticket that year.