Joanna of Castile (6 November 1479 – 12 April 1555), historically known as Joanna the Mad (Spanish: Juana la Loca), was the nominal queen of Castile from 1504 and queen of Aragon from 1516 to her death in 1555. She was the daughter of Queen Isabella I of Castile and King Ferdinand II of Aragon. Joanna was married by arrangement to the Austrian archduke Philip the Handsome on 20 October 1496. Following the deaths of her elder brother John, elder sister Isabella, and nephew Miguel between 1497 and 1500, Joanna became the heir presumptive to the crowns of Castile and Aragon. When her mother died in 1504, she became queen of Castile. Her father proclaimed himself governor and administrator of Castile.

In 1506, Joanna's husband Philip became king of Castile jure uxoris as Philip I, initiating the rule of the Habsburgs in the Spanish kingdoms. Philip died that same year. Despite being the ruling queen of Castile, Joanna had little effect on national policy during her reign as she was declared insane and confined in the Royal Palace in Tordesillas under the orders of her father, who ruled as regent until his death in 1516, when she inherited his kingdom as well. Her son Charles I became king, and during his reign Joanna was nominally co-monarch but remained confined until her death. Joanna died aged 75 in 1555, at which point her son Charles, the Holy Roman Emperor, became the sole ruler of Castile and Aragon.

Joanna was born on 6 November 1479 in the city of Toledo in the Kingdom of Castile. She was the fourth but third surviving child and the second daughter of Isabella I of Castile and Ferdinand II of Aragon, both members of the House of Trastámara.

She had a fair complexion and brown eyes, and her hair colour was between strawberry-blonde and auburn, like her mother and her sister Catherine. Her siblings were Isabella, Queen of Portugal; John, Prince of Asturias; Maria, Queen of Portugal; and Catherine, Queen of England.

Joanna was educated and formally trained for a significant marriage that, as a royal family alliance, would extend the kingdom's power and security as well as its influence and peaceful relations with other ruling powers. As an Infanta (princess), she was not expected to be heiress to the throne of either Castile or Aragon, although through deaths she later inherited both thrones.

Joanna's academic education consisted of canon and civil law, genealogy and heraldry, grammar, history, languages, mathematics, philosophy, reading, spelling and writing. Among the authors of classical literature she read were the Christian poets Juvencus and Prudentius, Church fathers Saint Ambrose, Saint Augustine, Saint Gregory, and Saint Jerome, and the Roman statesman Seneca.

In the Castilian court Joanna's main tutors were the Dominican priest Andrés de Miranda; educator Beatriz Galindo, who was a member of the queen's court; and her mother, the queen. Joanna's royal education included cooking, court etiquette, dancing, drawing, equestrian skills, music, and the needle arts of embroidery, needlepoint, sewing, spinning, and weaving. She studied the Iberian Romance languages of Castilian, Leonese, Galician-Portuguese and Catalan, and became fluent in French and Latin. She learned outdoor pursuits such as hawking and hunting. She was skilled at dancing and music; she played the clavichord, the guitar, and the monochord.

In 1496, 16-year-old Joanna was betrothed to 18-year-old Philip of Austria, in the Low Countries. Philip's parents were Maximilian I, Holy Roman Emperor, and his first wife, Duchess Mary of Burgundy. The marriage was one of a set of family alliances between the Habsburgs and the Trastámaras designed to strengthen both against growing French power.