Ferdinand I (of Aragón and Sicily), called The Just (c. 1379-1416), King of Aragón and Sicily (1412-1416). He was the younger son of John I, King of Castile, and Eleanor of Aragón.

In 1406, upon the death of his elder brother King Henry III of Castile, Ferdinand declined the Castilian crown and instead, with Henry's widow Catherine, became coregent during the minority of his nephew John II of Castile. In this capacity he distinguished himself by his prudent administration of domestic affairs.

After Ferdinand's uncle, Martin I of Aragon and, as Martin II, King of Sicily, died without issue, Ferdinand was chosen king in 1412 to succeed him.

The most notable accomplishment of his brief reign was his agreement in 1416 to depose the Antipope Benedict XIII, thereby helping to end the Great Schism, which had divided the Western Church for nearly 40 years.

In 1393, Ferdinand was married to Leonor Urraca de Castilla, condessa de Alburquerque (1374 -1435). They had 8 children: Alfonso V of Aragon; John II of Aragon; Enrique, duque de Villena; Pedro, duca di Noto; Sancho; Maria, first wife of John II of Castile; Leonor, who married Edward I of Portugal; Isabella,contesa de Urgel, who married Pedro, duke of Coimbra