Sir William Sidney Smith (July 21, 1764 - 1840), English admiral, was the second son of Captain John Smith of the Guards, and was born at Westminster.

He entered the navy, according to his own account, "at the beginning of the American War," being only about eleven years of age. For his bravery under Rodney in the action near Cape St Vincent in January 1780, he was on September 25 appointed lieutenant of the "Alcide." After serving in the actions against the French fought by Graves off Chesapeake in 1781 and by Rodney at the Leeward Islands in 1782, he was on May 6 of the latter year promoted to be commander of the "Fury" sloop, and on October 18 advanced to the rank of captain.

His ship having been paid off in the beginning of 1784, he spent two years in France and afterwards visited Spain. From 1790 to 1792 he advised the king of Sweden in the war with Russia, receivihg for his services the honour of knighthood.

After his return to England he was sent on a mission to Constantinople, and having joined Lord Hood at Toulon from Smyrna in December 1793, he, though only on half pay, was actively employed in the attempt to burn the enemy's ships and arsenal. In the following years he was engaged in the Channel hunting French privateers; but, having with the boats of his squadron boarded in Havre-de-Grace harbour a lugger which was driven by the tide above the French forts, he was on April 19 1796 compelled to surrender and sent a prisoner to Paris. By means of forged orders for his removal to another prison he made his escape from the Temple, and, crossing the Channel in a small skiff picked up at Havre, arrived in London on May 8, 1798.

In October he was appointed to the command of the "Tigre," and was sent to the Mediterranean. By a very curious decision of the government he was joined in commission with his brother Spencer Smith, minister at Constantinople. Learning of Bonaparte's approach to St Jean d'Acre, he hastened to its relief, and on March 16, 1799 captured the enemy's flotilla, after which he successfully defended the town, compelling Napoleon on May 20 to raise the siege and retreat in disorder, leaving all his artillery behind. For this brilliant exploit he received the special thanks of the Houses of Parliament and was awarded an annuity of £1000.

On January 24 1800 he took upon himself to make the convention of El Arish, by which the French were to have been allowed to evacuate Egypt. His action was disallowed by his superiors, who insisted that the French must surrender. Subsequently he co-operated with Abercromby, under whom he commanded the naval brigade at the battle of Aboukir, where he was wounded.

On his return to England he was in 1802 elected M.P. for the city of Rochester. In March 1803 he was commissioned to watch the preparations of the French for an invasion of England. Having on November 9, 1805 been promoted to be rear-admiral of the blue, he was in the following January despatched on secret service for the protection of Sicily and Naples. His conduct was as usual brilliant, but, also as usual, his vanity and self-assertion led him into quarrels with the military officers.