Thomas Addis Emmet (24 April 1764 – 14 November 1827) was an Irish and American lawyer and politician. In Ireland, in the 1790s, he was a senior member of the Society of United Irishmen as it planned for an insurrection against the British Crown and Protestant Ascendancy. In American exile, he took up legal practice in New York, earned a reputation as a staunch abolitionist, and in 1812 to 1813 served as the state's Attorney General.

Thomas Addis Emmet was born in the Hammond's Marsh area of Cork on 24 April 1764. He was a son of Dr. Robert Emmet from Tipperary town (later to become State Physician of Ireland) and Elizabeth Mason of County Cork, both of whose portraits are today displayed at Cork's Crawford Art Gallery. He was the elder brother of Robert Emmet, who was executed for leading the Irish Rebellion of 1803, becoming one of Ireland's most famous Republican martyrs. His sister, Mary Anne Holmes, held similar political beliefs.

Emmet was educated at Trinity College, Dublin and was a member of the committee of the College Historical Society. He later studied medicine at the University of Edinburgh and was a pupil of Dugald Stewart in philosophy. After visiting the chief medical schools on the continent, he returned to Ireland in 1788; but the sudden death of his elder brother, Christopher Temple Emmet (1761–1788), a student of great distinction, induced him to follow the advice of Sir James Mackintosh to forsake medicine for the law as a profession.

Emmet was a man of liberal political sympathies and became involved with the campaign to extend the democratic franchise for the Irish Parliament and to end discrimination against Catholics. He was called to the Irish bar in 1790 and quickly obtained a practice, principally as counsel for prisoners charged with political offences. This including defending James Napper Tandy. He also became the legal adviser of the Society of the United Irishmen, which included Tandy as a member.

When the Dublin Corporation issued a declaration of support of the Protestant Ascendancy in 1792, the response of the United Irishmen was their non-sectarian manifesto which was largely drawn up by Emmet. In 1795 he formally took the test or pledge of the United Irishmen (composed by his friend, William Drennan), becoming secretary in the same year and a member of the executive in 1797. By this time, the United Irishmen had been declared illegal and driven underground. Efforts to secure Catholic emancipation and parliamentary reform were abandoned in favour of preparations for a republican insurrection. At odds with Lord Edward Fitzgerald and other leading members, Emmet cautioned against proceeding in advance of a French landing.

British intelligence had infiltrated the United Irishmen and managed to arrest most of their leaders on the eve of the rebellion. Though not among those taken at the house of Oliver Bond on 12 March 1798 (see Lord Edward Fitzgerald), he was arrested about the same time, and was one of the leaders imprisoned initially at Kilmainham Jail and later in Scotland at Fort George until 1802. Upon his release, he went to Brussels where he was visited by his brother Robert Emmet in October 1802 and was informed of the preparations for a fresh rising in Ireland in conjunction with French aid. However, at that stage, France and Britain were briefly at peace, and the Emmets' pleas for help were turned down by Napoleon.

He received news of the failure of Robert Emmet's rising in July 1803 in Paris, where because of his alleged Jacobin sympathies and complaints regarding broken French promises he had lost out to Arthur O'Connor as the favoured Irish emissary. Disillusioned with Napoleon's new imperial regime, and convinced that if it invaded Ireland it would reduce it to another of its client states, on 4 October 1804 he sailed with his family for America.

On his arrival, he wrote to Robert Simms in Belfast: "France is the headquarters of fraud, deceit, despotism and under its present rulers no Nation or People who love liberty need look for its honest cooperation".