John Ponsonby, 1st Viscount Ponsonby, GCB (1770 – 22 February 1855) was a longtime British diplomat and politician.
Ponsonby, eldest son of the 1st Baron Ponsonby, and brother of Major-General Sir William Ponsonby, was born about 1770. He served as a Member of Parliament (MP) in the Irish House of Commons for Tallow between 1793 and 1797. Elected in 1798 for both Banagher and Dungarvan, he elected to sit for the latter from 1798 to the Act of Union in 1800/01. He then represented Galway Borough in the United Kingdom House of Commons until 1802.
On the death of his father on 5 November 1806, he succeeded him as Baron Ponsonby, and for some time held an appointment in the Ionian Islands. On 28 February 1826 he went to Buenos Aires as envoy-extraordinary and minister-plenipotentiary until 1828 and moved then to Rio de Janeiro in the same capacity. An exceptionally handsome man, he was sent, it was reported, to South America by George Canning to please George IV, who was envious of the attention paid him by Lady Conyngham. Once there he greatly fostered the creation of a buffer state between Argentina and Brazil to the benefit of British commerce: Uruguay. In 1830, he was entrusted with a special mission to Belgium on 1 December 1830, in connection with the candidature of Prince Leopold of Saxe-Coburg to the throne, and remained in Brussels until Leopold was elected king on 4 June 1831. His dealings with this matter were adversely criticised in ‘The Guet-à-Pens Diplomacy, or Lord Ponsonby at Brussels, …’ London, 1831. But Lord Grey eulogised him in the House of Lords on 25 June 1831. Thus, as a diplomat, he was sent twice by the British Empire to promote the instauration of buffer states to protect its interests, Uruguay and Belgium, both of which survive to this very day, still deeply similar to their bigger neighbours. In addition to this, Ponsonby was envoy to Naples from 8 June to 9 November 1832, ambassador at Constantinople from 27 November 1832 to 1841, and ambassador at Vienna from 10 August 1846 to 31 May 1850.