Alfonso IX (August 15, 1171 – September 23 or 24, 1230), first cousin of Alfonso VIII of Castile and numbered next to him as being a junior member of the family, is said by Ibn Khaldun to have been called the Baboso or Slobberer because he was subject to fits of rage during which he foamed at the mouth, was the king of León from the death of his father Ferdinand II in 1188 until his own death.

Alfonso was the only son of King Ferdinand II of León and Urraca of Portugal. Though he took a part in the work of the reconquest, this king is chiefly remembered by the difficulties into which his successive marriages led him with the pope. He was first married in 1191 to his cousin Teresa of Portugal, who bore him two daughters, and a son who died young.

The marriage was declared null by the pope, to whom Alfonso paid no attention till he was presumably tired of his wife. It cannot have been his conscience which constrained him to leave Teresa, for his next step was to marry Berenguela of Castile in 1197, who was his second cousin. For this act of contumacy the king and kingdom were placed under interdict.

The pope was, however, compelled to modify his measures by the threat that if the people could not obtain the services of religion they would not support the clergy, and that heresy would spread. The king was left under interdict personally, but to that he showed himself indifferent, and he had the support of his clergy. Berenguela left him after the birth of five children, and the king then returned to Teresa, to whose daughters he left his kingdom by will.

Alfonso's children by Teresa of Portugal were:
1. Sancha (c. 1192-1270)
2. Dulce, also called Aldonza (c. 1195-c.1243)
3. Fernando (1204-August 1214)

His eldest daughter, Sancha, was engaged to her cousin King Henry I of Castile, but Henry died in 1217 before the marriage could be solemnized. Wanting to disinherit his eldest son, Fernando, King Alfonso invited John of Brienne to marry his daughter Sancha and thus inherit the Leonese throne. However, Queen Berenguela convinced John of Brienne to marry one of her daughters instead. Though she was the nominal heiress on her father's death in 1230, Sancha was easily set aside by Berenguela and Fernando. Sancha became a nun at Cozollos, where she died in 1270; she was later beatified. Her sister Dulce-Aldonza spent her life with their mother in Portugal.

Alfonso's children by Berenguela of Castile were:
1. Leonor (1198/99-October 31, 1210)
2. King Fernando III (1200-1252)
3. Berenguela (1201-1237), married John of Brienne
4. Alfonso, Lord of Molina (1203-1272)
5. Constanza (1205-September 7, 1242), became a nun at Las Huelgas