James Napper Tandy (February 1739 – 24 August 1803), known as Napper Tandy, was an Irish revolutionary and a founder of the United Irishmen. He experienced exile, first in the United States and then in France, for his role in attempting to advance a republican insurrection in Ireland with French assistance.

A Dubliner, a Protestant (Church of Ireland), and the son of an ironmonger, Tandy was baptised (as 'James Naper Tandy') in St. Audoen's Church on 16 February 1739. He went to the famous Quaker boarding school in Ballitore, south Kildare, also attended by Edmund Burke, who was eight years older. He then started life as a small tradesman in Dublin's inner city. He was a churchwarden at St. Audoen's in 1765, and also at another local church (either St. Bride's or St. John's) where he commissioned a new church bell bearing his name, displayed since 1946 on the floor of St. Werburgh's Church.

Turning to politics, he was elected a member of Dublin Corporation representing the Guild of Merchants, and was popular for his denunciation of municipal corruption and his proposal of a boycott of English goods in Ireland in retaliation for the restrictions imposed by the Westminster government on Irish commerce.

In April 1780, Tandy was expelled from the Dublin Volunteers (see Henry Flood) for proposing the expulsion of the Duke of Leinster. He was one of the most conspicuous members of the small revolutionary party, chiefly of the shopkeeper class, that formed a permanent committee in June 1784 to agitate for reform, and called a convention of delegates from all parts of Ireland. This met in October 1784.

Tandy and John Binns persuaded Dublin Corporation to condemn by resolution Pitt's amended commercial resolutions in 1785. He became a member of the Whig Club founded by Henry Grattan, but he identified with its radical faction and actively co-operated with Theobald Wolfe Tone in founding the Society of the United Irishmen in 1791, of which he became the first secretary.

His opinions, strongly influenced by French Revolutionary ideas, now brought Tandy to the notice of the authorities. In February 1792, an allusion in a debate by John Toler, the Attorney General of Ireland, to Tandy's personal ugliness provoked him into sending a challenge to a duel. This was treated by the House of Commons as a breach of a Member's privilege, and a Speaker's warrant was issued for his arrest, which he managed to elude until its validity expired on the prorogation of Parliament. Tandy then took proceedings against the Lord Lieutenant for issuing a proclamation for his arrest; although the action failed, it increased Tandy's popularity, and his expenses were paid by the Society of the United Irishmen.

Sympathy with the French Revolution was rapidly spreading in Ireland. A meeting of some 6,000 people in Belfast voted for a congratulatory address to the French nation in July 1791. In the following year, Napper Tandy took a leading part in organising a new military association in Ireland modelled after the French National Guards; they professed republican principles, and on their uniform, the cap of liberty instead of the crown surmounted the Irish harp. Tandy also, with the purpose of bringing about a fusion between the Defenders and the United Irishmen, took the oath of the Defenders, a Roman Catholic society whose agrarian and political violence had been increasing for several years.

He was about to be tried in 1793 for distributing a seditious pamphlet in County Louth when the government found out he had taken the oath of the Defenders. Being threatened with prosecution for this step, and also for libel, he took refuge by changing his Dublin address often. In 1795 he fled to the United States, where he remained until 1798. In February of that year, he went to Paris, where a number of Irish refugees, the most prominent of whom was Wolfe Tone, were assembled, planning rebellion in Ireland to be supported by a French invasion, but quarrelling among themselves over tactics.