Joanna (November 6, 1479 – 12 April 1555), known as Joanna the Mad (Spanish: Juana la Loca), is queen of Castile from 1504 and of Aragon from 1516.

From the union of these two crowns modern Spain evolves.

Joanna marries Philip the Handsome, who is crowned King of Castile in 1506, initiating the rule of the Habsburgs in Spain.

After Philip's death this same year, Joanna is deemed mentally ill and is confined to a nunnery for the rest of her life.

Though she remains the legal queen of Castile throughout this time, her father, Ferdinand II of Aragon, is regent until his death, when she inherits his kingdom as well.

From 1517, her son, Charles, rules as king, while she nominally remains co-monarch.