Sir Alexander Armstrong KCB FRS (1818 – 4 July 1899), born in County Donegal, Ulster, Ireland, was a naval surgeon, explorer, and author who from 1850 to 1854 sailed the Arctic on HMS Investigator under the command of Robert McClure in search of the lost expedition of explorer Sir John Franklin. Armstrong's account of the voyage, Personal narrative of the discovery of the north-west passage, was published in London in 1857.
He was educated at Trinity College, Dublin, and at the University of Edinburgh. He entered the Medical Department of the Royal Navy in 1842 and became its Director-General in 1869. He was present during the naval bombardment of Sveaborg during the Crimean War. He became a Knight Commander of the Order of the Bath in 1871 and served as honorary physician to The Queen and to The Prince of Wales (later Edward VII).