Ferdinand III, the Saint, (1198/1199 - May 30, 1252) was a king of Castile (1217 - 1252) and Leon (1230 - 1252). He was the son of Alfonso IX and Berenguela of Castile, daughter of Alfonso VIII.

In 1231 he united Castile and Leon permanently.

Ferdinand spent much of his reign fighting the Moors. He captured the towns of Córdoba in 1236, Jaén in 1246, and Seville in 1248, and occupied Murcia in 1243, thereby completing the reconquest of Spain excepting Granada, whose king nevertheless did homage to Ferdinand.

In 1219, Ferdinand married the daughter of the German king Philip of Swabia, Elizabeth, by whom he had seven sons (Alfonso, Frederick, Ferdinand, Phillip, Henry, Sancho and Manuel) and three daughters (Eleonor, Berenguela and Mary). After Elizabeth died in 1235, he married Joan of Dammartin (or Ponthieu). They had two sons, Ferdinand and Louis, and one daughter, Eleanor of Castile, wife of King Edward I of England.

He founded the University of Salamanca and the Cathedral of Burgos.

Ferdinand was canonized by Pope Clement X in 1671.