John Henry Petty, 2nd Marquess of Lansdowne (6 December 1765 – 15 November 1809), known as Earl Wycombe between 1784 and 1805, was a British Whig politician who in Ireland was suspected of complicity in a republican conspiracy.

After witnessing revolutionary events in Paris, he began to establish an independent reputation as a friend of reform, critical of the war with France and of the suppression of democratic agitation at home.

Assured of his liberty by the Irish Chief Secretary, William Wickham, who privately confessed to his own sympathy for Emmet and his cause, Petty retired in ill health to England where, in possession of his father's title Marquess of Lansdowne, he died aged 43.

Wycombe began to establish an independent reputation on his return to England late in 1792, joining Charles Fox as an outspoken critic of his father's former protégé, William Pitt, now Tory prime minister.

Despairing of parliamentary reform in Ireland and in the hope of French assistance, under the leadership of the scion of another leading Anglo-Irish family, Edward Fitzgerald, they were preparing a republican insurrection.

[8] To Henry Vassall-Fox, Lord Holland, Petty reported on the martial-law reign of terror that marked the suppression of the United Irish risings in the summer of 1798: pillaging, floggings and summary executions.

[9] Wycombe is one a number of "persons of respectability" that the early historian of the United Irishmen, Richard Madden, records as subsequently coming under the "usual power of fascinating" exercised by Robert Emmet.

On 10 December 1803, he wrote to the Irish Chief Secretary William Wickham asking if a warrant of arrest had been issued in his name and was relieved to discover that this was "utterly unfounded".

[16] Wycombe's letters to Lady Frances reveal that he continued to entertain criticism of government policy in Ireland, including the Act of Union (which her step son, Lord Castlereagh, helped push though the Irish Parliament in 1800); of the Anglican church establishment with its tithes levied atop rack rents; of "British tyranny in navigation"; and of religion ("a bad substitute for common sense").

[17] On his death in 1809, he was succeeded as Marquess of Lansdowne by his half-brother Henry Petty-Fitzmaurice, who in the ministry of "All the Talents" under Lord Grenville had three years previously been made Chancellor of the Exchequer at the age of twenty-five.