Mary I (18 February 1516 – 17 November 1558), also known as Mary Tudor, was the queen of England and Ireland from July 1553 until her death. She is best known for her vigorous attempt to reverse the English Reformation, which had begun during the reign of her father, Henry VIII. She restored, to the Church, some of the property taken from it in the previous two reigns. She was not able to legislate to force those who, at that time, held property which had been plundered from the Catholic Church and the monasteries. During her five-year reign, Mary had over 280 religious dissenters burned at the stake in the Marian persecutions, which led to her denunciation as "Bloody Mary" by her Protestant opponents.

At her funeral service, John White, bishop of Winchester, praised Mary: "She was a king's daughter; she was a king's sister; she was a king's wife. She was a queen, and by the same title a king also." She was the first woman to successfully claim the throne of England, despite competing claims and determined opposition, and enjoyed popular support and sympathy during the earliest parts of her reign, especially from the Roman Catholics of England.

Mary's attempts to undo the reforming work of her brother's reign faced major obstacles. Despite her belief in the papal supremacy, she ruled constitutionally as the Supreme Head of the English Church, a contradiction under which she bridled. She found herself entirely unable to restore the vast number of ecclesiastical properties handed over or sold to private landowners.

Protestant writers at the time, and since, have often taken a highly negative view of Mary's reign. By the 17th century, the memory of her religious persecutions had led to the adoption of her sobriquet "Bloody Mary". John Knox attacked her in his First Blast of the Trumpet against the Monstrous Regiment of Women (1558), and she was prominently vilified in Actes and Monuments (1563), by John Foxe. Subsequent editions of Foxe's book remained popular throughout the following centuries and helped shape enduring perceptions of Mary as a bloodthirsty tyrant.

Mary is remembered in the 21st century for her vigorous efforts to restore the primacy of Roman Catholicism in England after the rise of Protestant influence during the short-lived reign of her half-brother, Edward. Protestant historians have long denigrated her reign, emphasizing that in just five years she burned several hundred Protestants at the stake in the Marian persecutions. In the mid-20th century, H. F. M. Prescott attempted to redress the tradition that Mary was intolerant and authoritarian, and scholarship since then has tended to view the older, simpler assessments of Mary with increasing reservations. A historiographical revisionism since the 1980s has to some degree improved her reputation among scholars. Christopher Haigh argued that her revival of religious festivities and Catholic practices was generally welcomed. Haigh concluded that the "last years of Mary's reign were not a gruesome preparation for Protestant victory, but a continuing consolidation of Catholic strength."

Catholic historians, such as John Lingard, thought Mary's policies failed not because they were wrong but because she had too short a reign to establish them and because of natural disasters beyond her control. In other countries, the Catholic Counter-Reformation was spearheaded by Jesuit missionaries; Mary's chief religious advisor, Cardinal Reginald Pole, refused to allow the Jesuits into England. Her marriage to Philip was unpopular among her subjects and her religious policies resulted in deep-seated resentment. The military loss of Calais to France was a bitter humiliation to English pride. Failed harvests increased public discontent. Philip spent most of his time abroad, while his wife remained in England, leaving her depressed at his absence and undermined by their inability to have children. After Mary's death, Philip sought to marry Elizabeth but she refused him.