Sir William Balfour (died 1660), of the family of Balfour of Pitcullo, Fife, Scotland, was a general of the parliamentary forces during the English Civil War.
Balfour appears to have been born before the accession of James I to the English throne in 1603, for in 1642 he obtained a naturalisation bill. He entered Dutch service in the Dutch States Army during the Thirty Years' War fighting with the Scottish brigade until 1627. In that year he became lieutenant-colonel in the Earl of Morton's regiment, took part in the expedition to the isle of Rhé to relieve the Siege of La Rochelle, and was noticed as being one of the officers most favoured by the Duke of Buckingham.
In January 1628 he was charged by Charles I, in conjunction with Colonel Dalbier, to raise 1,000 horse in Friesland, but the suspicions this project aroused in the House of Commons of England obliged the king to abandon the plan, and to assure the house that these troops were never meant to be employed in England. On the death of Sir Allen Apsley in 1630, Sir William, who is described as one of the gentlemen of the king's privy chamber, was appointed Lieutenant of the Tower of London.
In October 1631 he was employed on a confidential mission to the Netherlands. He also received many other marks of the king's favour, including the grant in 1633 of a lucrative patent for making gold and silver money in the Tower. Nevertheless Balfour, "from the beginning of the Long parliament, according to the natural custom of his country, forgot all his obligations to the king, and made himself very gracious to those people whose glory it was to be thought enemies of the court". Perhaps religious motives had something to do with this change of parties, for Balfour was a devout Presbyterian and a violent opponent of popery (as Roman Catholicism was called in England at the time), and had once beaten a priest for trying to convert his wife.