Mary I (18 February 1516 – 17 November 1558), also known as Mary Tudor, and as "Bloody Mary" by her Protestant opponents, was Queen of England and Ireland from July 1553 until her death in 1558. She is best known for her vigorous attempt to reverse the English Reformation, which had begun during the reign of her father, Henry VIII. Her attempt to restore to the Church the property confiscated in the previous two reigns was largely thwarted by Parliament, but during her five-year reign, Mary had over 280 religious dissenters burned at the stake in the Marian persecutions.

Mary was the only child of Henry VIII by his first wife, Catherine of Aragon, to survive to adulthood. Her younger half-brother, Edward VI, succeeded their father in 1547 at the age of nine. When Edward became mortally ill in 1553, he attempted to remove Mary from the line of succession because he supposed, correctly, that she would reverse the Protestant reforms that had taken place during his reign. Upon his death, leading politicians proclaimed Lady Jane Grey as queen. Mary speedily assembled a force in East Anglia and deposed Jane, who was ultimately beheaded. Mary was—excluding the disputed reigns of Jane and the Empress Matilda—the first queen regnant of England. In July 1554, Mary married Philip of Spain, becoming queen consort of Habsburg Spain on his accession in January 1556.

After Mary's death in 1558, her re-establishment of Roman Catholicism was reversed by her younger half-sister and successor, Elizabeth I.