Thomas Blood (1618 – 24 August 1680) was an Anglo-Irish army officer and self-styled colonel best known for his attempt to steal the Crown Jewels of England from the Tower of London in 1671. Described in an American source as a "noted bravo and desperado," he was also known for his attempt to kidnap and, later, to kill his enemy, the 1st Duke of Ormond.

Sources suggest that Blood was born in County Clare, in the Kingdom of Ireland, the son of a successful land-owning blacksmith of English descent, and was partly raised at Sarney, near Dunboyne, in County Meath. He was apparently a Presbyterian. His family was respectable and prosperous (by the standards of the time); his father held lands in Counties Clare, Meath and Wicklow. His grandfather was a member of the Irish Parliament, and had lived at Kilnaboy Castle (also in County Clare). He received his education in Lancashire, England. At the age of 20, he married Maria Holcroft, the daughter of John Holcroft of Holcroft Hall, Culcheth, Cheshire, and Golborne, Lancashire, and returned to Ireland.

At the outbreak of the First English Civil War in 1642, Blood returned to England and initially took up arms with the Royalist forces loyal to Charles I. As the conflict progressed he switched sides and became a lieutenant in Oliver Cromwell's Roundheads. In 1653 at the cessation of hostilities Cromwell awarded Blood land grants as payment for his service and appointed him a justice of the peace. Following the Restoration of King Charles II to the Crowns of the Three Kingdoms in 1660, Blood fled with his family to Ireland. The confiscations and restitutions under the Act of Settlement 1662 (which sought to cancel and annul some of the grants of land and real properties allocated as reward to new holders being Cromwellians under the Act of Settlement 1652) brought Blood to financial ruin, and in return Blood sought to unite his fellow Cromwellians in Ireland to cause insurrection.

As part of the expression of discontent, Blood conspired to storm Dublin Castle, usurp the government, and kidnap for ransom the 1st Duke of Ormond, who was the Lord Lieutenant of Ireland. On the eve of the attempt, the plot was foiled. Blood managed to evade the authorities by hiding with his countrymen in the mountains, and ultimately escaped to the United Dutch Provinces in the Low Country. A few of Blood's collaborators were captured and executed. As a result, some historians speculated that Blood swore vengeance against Ormond.

While in the Dutch Republic, Blood gained the favour of Admiral de Ruyter, an opponent of the English forces in the Anglo-Dutch Wars, and was implicated in the Scottish Pentland Rising of 1666 by the Scottish Presbyterian Covenanters. At some point during this period, Blood became associated with the wealthy George Villiers, 2nd Duke of Buckingham, who 19th-century commentators believed used Blood as a means to punish his own political and social adversaries, since his own class ranking did not allow him to meet them "in the field".

In 1670, despite his status as a wanted man, Blood returned to England and is believed to have taken the name Ayloffe and practised as a doctor or an apothecary in Romford Market, east of London. A second attempt, this time on the life of the Duke of Ormond, followed.

Since Ormond's return to England, he had taken up residence at Clarendon House. Blood had followed Ormond's movements and noted that he frequently returned late in the evening accompanied by a small number of footmen. On the night of 6 December 1670, Blood and his accomplices attacked Ormond while the latter travelled St James's Street. Ormond was dragged from his coach, bound to one of Blood's henchmen, and taken on horseback along Piccadilly with the intention of hanging him at Tyburn. The gang pinned a paper to Ormond's chest spelling out their reasons for his capture and murder. With one of his servants who had given chase on horseback, Ormond succeeded in freeing himself and escaped. The plot's secrecy meant that Blood was not suspected of the crime, despite a reward being offered for the capture of the attempted assassins. In the King's presence, James's son, Thomas Butler, accused the Duke of Buckingham of being behind the crime. Thomas threatened to shoot Buckingham dead in revenge, if his father, James, was murdered.

Blood did not lie low for long, and within six months he made his notorious attempt to steal the Crown Jewels. In April or May 1671 he visited the Tower of London dressed as a parson and accompanied by a female companion pretending to be his wife. The Crown Jewels could be viewed by the payment of a fee to the custodian. While viewing the Crown Jewels, Blood's "wife" feigned a stomach complaint and begged the newly appointed Master of the Jewel House, 77-year-old Talbot Edwards, to fetch her some spirits. Given the proximity of the jewel keeper's domestic quarters to the site of the commotion, Edwards' wife invited them upstairs to their apartment to recover, after which Blood and his wife thanked the Edwardses and left.