Sir William Fanshawe Martin (December 5, 1801 - March 24, 1895), was a British admiral.

He was the son of Admiral of the Fleet Sir Thomas Byam Martin , comptroller of the navy, and grandson, on the mother's side, of Captain Robert Fanshawe , who commanded the "Namur 90" in George Rodney's victory of April 12, 1782.

Entering the navy at the age of twelve, his fathers interest secured his rapid promotion: he was made a lieutenant on December 15, 1820; on February 8, 1823 he was promoted to be commander of the "Fly" sloop, his good service in which in support of the interests of British merchants at Callao secured his promotion as captain on June 5, 1824. He afterwards served in the Mediterranean and on the home station.

In 1849-1852 he was commodore commanding the Channel squadron, and gave evidence of a remarkable aptitude for command. He was made rear-admiral in May 1853, and for the next four years was superintendent of Portsmouth dockyard. He was made vice-admiral in February 1858, and after a year as a lord of the admiralty, was appointed commander-in-chief in the Mediterranean Sea.

The discipline of the navy was then bad. It was a tradition sprung from the wholesale shipment of gaol-birds during the old war, that the men were to be treated without consideration; moreover the ships had been largely filled up with bounty men bought into the service with a £10 note, without training. Out of this unpromising material Martin formed the fleet which was at that time the ideal of excellence.

He had no war service, and, beyond the Italian disturbance of 1860-61, no opportunity for showing diplomatic ability. But his memory lives as that of the reformer of discipline and the originator of a comprehensive system of steam manoeuvres. He became an admiral in November 1863, and on the 4th of December succeeded to the baronetcy which had been conferred on his grandfather.

His last appointment was the command at Plymouth, 1866-1869, and in 1870 he was put on the retired list. In 1873 the GCB was conferred on him, and in 1878 he was made rear-admiral. He died at Upton Grey, near Winchfield , on the 24th of March 1895. He was twice married, and left, besides daughters, one son, who succeeded to the baronetcy.