Sir William Pelham (c. 1528 – 1587) was an English soldier and Lord Justice of Ireland, a military and political role rather than a judicial one.
He was third son of Sir William Pelham of Laughton, Sussex, by his second wife, Mary, daughter of William Sandys, 1st Baron Sandys of the Vyne near Basingstoke in Hampshire. His father died in 1538, and Pelham was probably thirty when he was appointed captain of the pioneers at the siege of Leith in 1560. Among the siegeworks, his pioneers built a sconce with four bastions which was called "Mount Pelham." William was commended on that occasion; but, according to Humfrey Barwick, his bad engineering was responsible for the wound inflicted during the assault on Arthur Grey, 14th Lord Grey de Wilton, the son of the army's commander.
He commanded the pioneers at Le Havre in November 1562 under Ambrose Dudley, 3rd Earl of Warwick; and, despatched to the assistance of Admiral Coligny in February 1563, was present at the capture of Caen. Returning to Le Havre in March, he was wounded during a skirmish with the forces of the Rhinegrave in June. He assisted at the negotiations for the surrender of Le Havre, and was a hostage for the fulfilment of the conditions of surrender. Subsequently, on his return to England, he was employed with Portinari and Jacopo Aconcio in inspecting and improving the fortifications of Berwick upon Tweed. Confidence was reposed in his judgment, and, appointed lieutenant-general of the ordnance, he was chiefly occupied for several years in strengthening the defences of the kingdom. He accompanied Henry, lord Cobham, and Secretary Francis Walsingham on a diplomatic mission to the Netherlands in the summer of 1578, and in the following summer he was sent to Ireland to organise the defence of the Pale against possible inroads by the O'Neills. He was knighted by Sir William Drury, and, on the latter's death shortly afterwards, was chosen by the council lord justice ad interim.