Mary I (18 February 1516 – 17 November 1558) was Queen of England and Queen of Ireland from 19 July 1553 until her death. She was the eldest daughter of Henry VIII and only surviving child of Catherine of Aragon. As the fourth crowned monarch of the Tudor dynasty, she is remembered for restoring England to Roman Catholicism after succeeding her short-lived half brother, Edward VI, to the English throne. In the process, she had almost 300 religious dissenters burned at the stake in the Marian Persecutions, earning her the sobriquet of "Bloody Mary". Her re-establishment of Roman Catholicism was reversed by her successor and half-sister, Elizabeth I.

Mary was the only child of Henry VIII and his first wife Catherine of Aragon to survive infancy. Through her mother, she was a granddaughter of Ferdinand II of Aragon and Isabella I of Castile. She was born at the Palace of Placentia in Greenwich, London. Mary was a sickly child who had poor eyesight, sinus conditions and bad headaches. Despite her problems, Mary was a precocious child. A great part of her early education came from her mother, who consulted the Spanish humanist Juan Luis Vives for advice and was herself Mary's first instructor in Latin. Mary also studied Greek, science, and music. When Mary was nine years old, Henry gave her her own court at Ludlow Castle and many of the Royal Prerogatives normally only given to a Prince of Wales, even calling her the Princess of Wales. Despite this obvious affection, Henry was deeply disappointed that his marriage had produced no sons.

Throughout Mary's childhood, Henry negotiated potential future marriages for her. Meanwhile, the marriage of Mary's parents was in jeopardy because Catherine had failed to provide Henry the male heir he desired. Henry attempted to have his marriage to her annulled, but Pope Clement VII refused his requests. Some contend that the Pope's decision was influenced by Charles V, Mary's former betrothed and her mother's nephew. Henry had claimed, citing biblical passages, that his marriage to Catherine was unclean because she had been previously married briefly, at age 16 to his brother Arthur, although there was some debate as to whether that marriage had been consummated. In 1533, Henry married another woman, Anne Boleyn, and shortly thereafter, Thomas Cranmer, the Archbishop of Canterbury, formally declared the marriage with Catherine void and the marriage with Anne valid. Henry then broke with the Roman Catholic Church and declared himself head of the Church of England. As a consequence, Catherine was demoted to Dowager Princess of Wales (a title she would have held as the widow of Arthur). Mary in turn was deemed illegitimate, and her place in the line of succession transferred to her half-sister, the future Elizabeth I, daughter of Anne Boleyn. She was styled "Lady Mary" rather than princess because of her illegitimate status.

Mary was expelled from Court, her servants (including her favourite maid Susan Clarencieux) were dismissed from her service, and in December 1533 she was sent to serve as a lady-in-waiting to Elizabeth. Upon arriving at the house she was asked by the Duke of Norfolk if she would not go and pay her respects to the Princess to which Mary curtly replied that she knew of no princess in England save herself but as the king had acknowledged Elizabeth to be his, she might call her sister, as she called the Duke of Richmond (Henry's son by Elizabeth Blount) brother. Despite the cold treatment she received at Hatfield, Mary was also determined to assert her seniority over Elizabeth. Despite her courage and determination, Mary was often sick. Mary was not permitted to see her mother or attend her funeral in 1536. Circumstances between Mary and her father worsened, and she attempted to reconcile with him by submitting to his authority as head of the Church of England. By this she repudiated papal authority, acknowledged that the marriage between her mother and father was unlawful, and accepted her own illegitimacy. When Anne Boleyn was beheaded in 1536, Elizabeth was downgraded to the status of Lady and removed from the line of succession. Within two weeks of Anne Boleyn's execution, Henry married Jane Seymour, who died shortly after giving birth to a son, the future Edward VI. Mary was godmother to her half-brother Edward and chief mourner at Jane Seymour's funeral. In return, Henry agreed to grant her a household (which included the reinstatement of Mary's favourite maid Susan Clarencieux), and Mary was permitted to reside in royal palaces.

In 1543 Henry married his sixth and last wife, Catharine Parr, who was able to bring the family closer together. The next year, through the Third Succession Act, Henry returned Mary and Elizabeth to the line of succession, placing them after Edward. However, both women remained legally illegitimate.

In 1547, Henry died and was succeeded by his son, Edward VI. Since Edward was still a child, rule passed to a regency council dominated by Protestants, who attempted to establish their faith throughout the country. As an example, the Act of Uniformity 1549 prescribed Protestant rites for church services, such as the use of Thomas Cranmer's new Book of Common Prayer. When Mary, who had remained faithful to Roman Catholicism, asked to be allowed to worship in private in her own chapel, she was refused. It was only after Mary appealed to her cousin Charles V that she was allowed to worship privately. Religious differences continued to be a problem between Mary and Edward, however. When Mary was in her thirties, she attended a reunion with Edward and Elizabeth for Christmas, where Edward embarrassed Mary and reduced her to tears in front of the court for "daring to ignore" his laws regarding worship.

On 6 July 1553, at the age of 15, Edward VI died of tuberculosis. Edward did not want the crown to go to Mary, who he feared would restore Catholicism and undo his reforms, as well as those of Henry VIII. For this reason, he planned to exclude her from the line of succession. However, his advisors told him that he could not disinherit only one of his sisters, but that he would have to disinherit Elizabeth as well, even though she embraced the Church of England. Guided by John Dudley, 1st Duke of Northumberland and perhaps others, Edward excluded both of his sisters from the line of succession in his will. Edward VI and his advisors instead devised that he should be succeeded by Dudley's daughter-in-law Lady Jane Grey, the granddaughter of Henry VIII's younger sister Mary, the French Queen. However, this exclusion contradicted the Act of Succession of 1544. This act had restored Mary and Elizabeth to the line of succession. Around the time of Edward VI's death, Mary had been summoned back to London from Framlingham Castle in Suffolk, into which she had recently moved after having left her former residence at the Palace of Beaulieu. However, Mary initially hesitated; she suspected that this summons could be a pretext on which to capture her and, in so doing, facilitate Grey's accession to the throne. On 10 July 1553, Lady Jane Grey assumed the throne as Queen of England in what can best be described as a coup d'état orchestrated by Dudley and his supporters. As soon as Mary received the news, she fled to East Anglia, where Dudley had ruthlessly put down the Ket rebellion. Many conservatives lived there, who felt an instant loyalty to the daughter of Henry VIII. Dudley's support collapsed almost immediately, which led to the false Queen being deposed on July 22, 1553. Mary rode triumphantly into London on a wave of popular support to legally assume the crown. Grey and Dudley were immediately imprisoned in the Tower of London. Although Mary understood that the young Lady Jane was essentially a pawn in Dudley's scheme, she still had her 16 year old cousin executed.

One of Mary's first actions as Queen was to order the release of the Roman Catholic Thomas Howard, 3rd Duke of Norfolk and Stephen Gardiner from imprisonment in the Tower of London. At this time, Dudley was the only conspirator of rank executed for high treason. Mary was left in a difficult position, as almost all the Privy Counsellors had been implicated in the plot to put Jane on the throne. She could only rely on Gardiner, whom she appointed both Bishop of Winchester and Lord Chancellor. On 1 October 1553, Gardiner formally crowned Mary. At age 37, Mary turned her attention to finding a husband and producing an heir, thus preventing the Protestant Elizabeth (still her successor under the terms of Henry VIII's will and more importantly the Act of Succession of 1544), from succeeding to the throne. Mary rejected Edward Courtenay, 1st Earl of Devon, as a prospect when her cousin Charles V suggested she marry his only son, the Spanish Prince Philip, later Philip II of Spain. It is said that upon viewing the Titian full-length portrait of Philip now in the Prado, which had been sent to her, Mary declared herself to be in love with him. Their marriage at Winchester Cathedral on 25 July 1554 took place just two days after their first meeting. Philip's view of the affair was entirely political, and it was extremely unpopular with the English. Lord Chancellor Gardiner and the House of Commons petitioned her to consider marrying an Englishman, fearing that England would be relegated to a dependency of Spain. This fear may have arisen from the fact that Mary was – excluding the brief, unsuccessful and controversial reigns of Jane and Empress Matilda – England's first Queen regnant. Under the terms of the marriage treaty, Philip was to be styled "King of England", all official documents (including Acts of Parliament) were to be dated with both their names, and Parliament was to be called under the joint authority of the couple. Coins were also to show the heads of both Mary and Philip. The marriage treaty further provided that England would not be obliged to provide military support to Philip's father in any war. In order to elevate his son to Mary's rank, Emperor Charles V ceded the crown of Naples, as well as his claim to the Kingdom of Jerusalem, to Philip. Therefore, Mary I became Queen of Naples and titular Queen of Jerusalem upon marriage. In 1556, Mary's father-in-law abdicated and she became Queen of Spain. Mary ruled England for five years. After that, Elizabeth succeeded her. Insurrections broke out across the country when she insisted on marrying Philip, with whom she was in love. The Duke of Suffolk once again proclaimed that his daughter, Lady Jane Grey, was queen. In support of Elizabeth, Thomas Wyatt led a force from Kent that was not defeated until he had arrived at London. After the rebellions were crushed, the Duke of Suffolk, his daughter, Lady Jane Grey, and her husband were convicted of high treason and executed. Elizabeth, though protesting her innocence in the Wyatt affair, was imprisoned in the Tower of London for two months, then was put under house arrest at Woodstock Palace.

Mary, thinking she was pregnant, had thanksgiving services at the diocese of London in November 1554. This turned out to be the first of two phantom pregnancies. Various theories have been put forward to explain her condition, including cysts or a psychological problem. Philip persuaded his wife to permit Elizabeth's release from house arrest, probably so that he would be viewed favourably by her in case Mary died in childbirth. Soon after the disgrace of the false pregnancy, Philip headed off to Flanders to command his armies against France. Mary was heartbroken and gradually fell into deep depression.

As Queen, Mary was very concerned about heresy and the English church. She had always rejected the break with Rome instituted by her father and the establishment of Protestantism by Edward VI. She had England reconcile with Rome and Reginald Cardinal Pole, the son of her governess the Countess of Salisbury (who was beheaded for treason by Mary's father Henry VIII) and once considered a suitor, became Archbishop of Canterbury; Mary had his predecessor Thomas Cranmer burned at the stake. Mary came to rely greatly on Pole for advice. Edward's religious laws were abolished by Mary's first Parliament in the Statute of Repeal Act (1553). Church doctrine was restored to the form it had taken in the 1539 Six Articles. Mary also persuaded Parliament to repeal the Protestant religious laws passed by Henry VIII. The Revival of the Heresy Acts were also passed in 1554. Numerous Protestants were executed in the Marian Persecutions. The persecution lasted for almost four years. All told, 284 were executed, most by burning.

Henry VIII's creation of the Kingdom of Ireland in 1542 was not recognised by Europe's Catholic powers. In 1555 Mary obtained a papal bull confirming that she and Philip were the monarchs of Ireland, and thereby the Church accepted the personal link between the kingdoms of Ireland and England. Furthering the Tudor Reconquest of Ireland, the midlands counties of Laois and Offaly were shired and named after the new monarchs respectively as "Queen's County" and "King's County". Their principal towns were respectively named Maryborough (now Portlaoise) and Philipstown (now Daingean). Under Mary's reign, English colonists were settled in the Irish midlands to reduce the attacks on the Pale (the colony around Dublin).

Having inherited the Spanish throne upon his father's abdication, Philip returned to England from March to July 1557 to persuade Mary to support Spain in a war against France (the Italian Wars). There was much opposition to declaring war on France. There existed an old alliance between Scotland and France; French trade would be jeopardised; and England had a distinct lack of finances because of a bad economic legacy from Edward VI's reign. As a result of her agreement to declare war (which violated the carefully-written marriage treaty), England became full of factions and seditious pamphlets of Protestant origin inflaming the country against the Spaniards. English forces fared badly in the conflict and as a result lost Calais, England's sole remaining continental possession, on 13 January 1558. Although this territory had recently become financially burdensome, the effects of its loss were ideological.

The most prominent problem was the decline of the Antwerp cloth trade. Despite Mary's marriage to Philip, England did not benefit from their enormously lucrative trade with the New World. The Spanish guarded their trading revenue jealously, and Mary could not condone illegitimate trade (in the form of piracy) because she was married to a Spaniard. In an attempt to increase trade and rescue the English economy, Mary continued Northumberland's policy of seeking out new commercial ports outside Europe.

Financially, Mary was trying to reconcile between a modern form of government — with correspondingly higher spending — and a medieval system of collecting taxation and dues. A failure to apply new tariffs to new forms of imports meant that a key source of revenue was neglected. In order to solve this problem, Mary's government published the "Book of Rates" (1558), listing the tariffs and duties for every import. This publication was not reviewed until 1604. Mary also appointed William Paulet, 1st Marquess of Winchester as Surveyor of Customs and assigned him to oversee the revenue collection system. Mary also started currency reform to counteract the dramatic devaluation overseen by Thomas Gresham that had characterised the last few years of Henry's reign and the reign of Edward VI. However, these measures were largely unsuccessful.

Mary decreed in her will that her husband should be the regent during the minority of her child. However, no child was born, and Mary died at age 42 at St. James's Palace on 17 November 1558. She was succeeded by her half-sister, who became Elizabeth I. Although her will stated that she wished to be buried next to her mother, Mary was interred in Westminster Abbey on 14 December in a tomb she eventually shared with Elizabeth.