Sir Henry Sidney KG (20 July 1529 – 5 May 1586) was an English soldier, politician and Lord Deputy of Ireland. Instrumental in the Tudor conquest of Ireland, his attempts to consolidate English power in Ireland were a major contributing factor to the Desmond Rebellions.

Sidney was the eldest son of Sir William Sidney of Penshurst (1482 – 11 February 1553) and Anne Pakenham (1511 – 22 October 1544). His father was a prominent politician and courtier during the reigns of Henry VIII and Edward VI, from both of whom he received extensive grants of land, including the manor of Penshurst in Kent, named Penshurst Place, which became the principal residence of the family.

Sidney was brought up at court as the companion of Prince Edward, later Edward VI, and he continued to enjoy the favour of the Crown, serving under Mary I of England and then, particularly, throughout the reign of Queen Elizabeth I.

In 1556, Sidney served in Ireland with the Lord Deputy, Thomas Radclyffe, 3rd Earl of Sussex, who in the previous year had married his sister Frances. Both served Queen Mary until her death in 1558. Sidney played a large part in expanding the English administration in the country, which had shrunk over the centuries to the area around Dublin known as the Pale. He was also involved in the civil and military measures taken by his brother-in-law for bringing Irish chieftains into submission to the English Crown, known as Surrender and Regrant.

In the course of the Lord Deputy's expedition to Ulster in 1557, Sidney devastated the island of Rathlin. In the following year, during the absence of Sussex in England, he had sole responsibility for the government of Ireland and conducted himself with marked ability. A second absence of the Lord Deputy from Ireland, on the accession of Queen Elizabeth, threw the chief control into Sidney's hands at the outbreak of trouble with Shane O'Neill, and he displayed great skill in temporising with the chieftain until Sussex reluctantly returned to his duties in August 1559. About the same time, Sidney resigned his office of Vice-Treasurer of Ireland on his appointment as president of the council of the Marches in Wales, and for the next few years, he resided chiefly at Ludlow Castle, with frequent visits to the court in London.

Queen Elizabeth sent Sidney to Scotland in July 1562. He was instructed to defer a meeting between Elizabeth and Mary, Queen of Scots, to the next year. The Scottish queen was unhappy at the news and wept.

While he was with Mary, Queen of Scots, in the garden of Holyrood Palace there was an embarrassing incident. A Captain Hepburn came up to the queen and handed her a paper while she was talking to Sidney. She passed it to her brother, James Stewart, then Earl of Mar who opened it to discover four stanzas of obscene verse and a pornographic drawing. Meanwhile, Hepburn had fled to England. Mary was particularly affronted that Hepburn's intervention occurred during her meeting with Sidney.

In 1565, Sidney was appointed Lord Deputy of Ireland in place of Sir Nicholas Arnold, who had succeeded the Earl of Sussex in the previous year. He said he found the English Pale to be in a more impoverished and turbulent condition than when he left it, and claimed the chief disturbing factor to be Shane O'Neill, Chief of the Name of Clan O'Neill. With difficulty he persuaded Elizabeth to sanction vigorous measures against O'Neill; and although the latter avoided a pitched battle, Sidney restored O'Neill's rival Calvagh O'Donnell to his rights, and established an English garrison at Derry to prevent O'Neill expanding his influence.