Admiral John Rivett-Carnac or John Rivett Carnac (27 June 1796 – 1 January 1869) was an officer in the Royal Navy who became an early explorer in Western Australia. He later attained the rank of admiral.
John Rivett-Carnac was born in Bombay, India on 27 June 1796. He was the seventh and youngest son of the eleven children of James Rivett (1759-1802) of the East India Company (EIC)by his wife Henrietta Fisher (1765- 1837), daughter of James Fisher, of Bombay. His eldest brother was James Rivett-Carnac who would become the Sir James Rivett-Carnac, 1st Bt., and a Governor of Bombay. His father assumed the name of Rivett-Carnac by Sign Manual on 17 Jul 1801 in compliance with the last will and testament of General John Carnac. General Carnac had married his father's sister Elizabeth and in 1776 his father went to live with them in Bombay, where he remained until his death in 1802. John Rivett-Carnac emigrated to England with his mother in 1804, two years after his father's death.
He entered the Royal Naval College at Portsmouth in 1810, and in 1813 was appointed midshipman on the 38-gun Junon. He saw action in the War of 1812, taking part in operations under Sir George Cockburn and Lieutenant Philip Westphal. In October 1814 he joined the 38-gun Sybille, with whom he went to Greenland to search for the American Commodore John Rodgers. He later transferred to the 74-gun Berwick, then the 98-gun Boyne, and finally the 100-gun Queen Charlotte. After passing his Navy examinations in May 1816, he was involved in the Bombardment of Algiers. From 1816 to 1818, Rivett-Carnac served as an admiralty-midshipman on the Inconstant, and later on the Vengeur. One 1 October 1818 he was promoted to lieutenant in the Albion. In January 1819 he joined the Racehorse, and in November 1821 transferred to the Rochfort. He served on that ship until August 1825, when he was transferred to the Galatea. On 23 January 1826, John Rivett-Carnac joined HMS Success under Captain (later Admiral Sir) James Stirling as First Lieutenant. Three months later he married Maria Jane Davis, daughter of EIC director and orientalist Samuel Davis at St. Marylebone, London.