Lieutenant-general John Fremantle CB (? – 1845) was a British Army officer of the Napoleonic Era.
He was the eldest son of Colonel Stephen Fremantle, by Albinia, daughter of Sir John Jeffrys, Bart., and after his father died when he was four, he was mentored by his uncle William. He joined the 2nd Foot Guards on 17 October 1805 following an education at the Royal Military College, High Wycombe and Lüneburg University. He joined the army in Bremen as an ensign under Lord Cathcart.
He took part in the 1806-07 expedition to seize Buenos Aires from the Spanish Empire as aide-de-camp (ADC) to General John Whitelocke where he was taken prisoner while fighting with the Rifle Corps.
Promoted to Lieutenant and Captain on 2 August 1810, as private secretary he accompanied Lord Howden to Lisbon where he served in most of the actions of the Peninsular War as adjutant until 1812 when he was appointed extra ADC. During the war he carried despatchs to England announcing the battles of Vitoria (1813) and Orthez (1814)
In 1813 he was appointed ADC and private secretary to the Duke of Wellington and was present at the Battle of Waterloo.