Admiral Sir William Fanshawe Martin, 4th Baronet GCB (5 December 1801 – 24 March 1895), was a Royal Navy officer. As a commander, he provided valuable support to British merchants at Callao in Peru in the early 1820s during the Peruvian War of Independence. He became First Naval Lord in the Second Derby ministry in March 1858 and in that capacity acted as a strong advocate for the procurement of Britain's first ironclad warship. He went on to be Commander-in-Chief, Mediterranean Fleet and in that role provided important assistance during the Italian disturbances in 1860 and 1861, reformed the system of discipline in his fleet and developed a comprehensive system of manoeuvres for steam ships.