Jonathan Hornblower (Chacewater, 5 July 1753 – Penryn, 23 February 1815) was a British pioneer of steam power.
The son of Jonathan Hornblower the Elder and the brother of Jabez Carter Hornblower, two fellow pioneers, the young Hornblower was educated at Truro Grammar School, then apprenticed to a metal-working tradesman at Penryn. He was baptized at Trelever on 25 July 1773.
He developed a compound engine in 1781 but was prevented from pursuing his invention by litigation with James Watt (Boulton & Watt) over intellectual property. His compound steam engine principle was not revived until 1804 by Arthur Woolf. Following the expiration of Boulton and Watt's patent, Hornblower's compound steam engine principle contributed significantly to the increases in steam engine efficiency.
He invented the double-beat steam valve in around 1800.
In other developments in steam technology, however, he was more successful and he died a rich man at Penryn in 1815. He is buried in St Gluvias churchyard.
He had married twice, firstly to Rosamund Phillips in 1775 and afterwards to Elizabeth Jordan with whom he had two surviving daughters, Rosamund (1789) and Elizabeth (1790).