Thomas Radclyffe (or Ratclyffe), 3rd Earl of Sussex KG (c. 1525 – 9 June 1583), was Lord Deputy of Ireland during the Tudor period of English history, and a leading courtier during the reign of Elizabeth I.

[1] Radclyffe's prominence in the kingdom was shown by his inclusion among the signatories to the letters patent of 16 June 1553 settling the crown on Lady Jane Grey as Edward's successor.

According to the Encyclopædia Britannica Eleventh Edition, the "prevailing anarchy in Ireland, a country which, nominally subject to the English Crown, was torn by feuds among its practically independent native chieftains, rendered the task of the lord deputy one of no ordinary difficulty; a difficulty that was increased by the ignorance of English statesmen con- cerning Ireland and Irish conditions, and by their incapacity to devise or to carry into execution any consistent and thorough- going policy for bringing the half-conquered island under an orderly system of administration.

[5] Having defeated O'Neill and his allies, the MacDonnells, the lord deputy, who by the death of his father in February 1557 became Earl of Sussex, returned to Dublin, where he summoned a parliament in June of that year.

Statutes were passed declaring the legitimacy of Mary I of England as Queen of the Kingdom of Ireland, reviving the laws for the suppression of heresy and forbidding the immigration of Scots.

He burnt farms and houses including Saddell, a castle of James MacDonnell or MacDonald of Dunyvaig and Glynnes (died 1565), and then marched south to burn Dunaverty and Machrimore.

[2] Far from being reluctant to employ scorched earth tactics because of the high civilian mortality that it wrought (as has been claimed elsewhere), the government forces resorted to land and crop-burning repeatedly during the mid-Tudor and early Elizabethan years, and did so precisely because it promised to wreak the most havoc, and kill the most people.

Shane O'Neill refused to meet Sussex without security for his safety, and having established his power in Ulster he demanded terms of peace which Elizabeth was unwilling to grant.

He was preparing for a fresh attempt when he was superseded by Gerald FitzGerald, 11th Earl of Kildare, who was commissioned by Elizabeth to open negotiations with O'Neill, the result of which was that the latter repaired to London and made a formal submission to the queen.

He does not appear to have incurred Elizabeth's displeasure, for in 1566 and the following year she employed him in negotiations for bringing about a different matrimonial alliance which he warmly supported, the proposal that she should bestow her hand on the Archduke Charles.

When this project failed, Sussex returned from Vienna to London in March 1568, and in July he was appointed Lord President of the North, a position which threw on him the responsibility of dealing with the rebellion of the Northern Earls of Northumberland and Westmorland in the following year.