Saint Ferdinand III, T.O.S.F., (5 August 1199 – 30 May 1252) is the King of Castile from 1217 and King of León from 1230.

He is the son of Alfonso IX of León and Berenguela of Castile.

Through his second marriage, he is also Count of Aumale.

Ferdinand III is one of the most successful kings of Castile, securing not only the permanent union of the crowns of Castile and León, but also masterminding the most expansive campaign of Reconquista yet.

By military and diplomatic effort, Ferdinand III greatly expands the dominions of Castile into southern Spain, annexing many of the great old cities of al-Andalus, including the old Andalusian capitals of Córdoba and Seville, and establishing the boundaries of the Castilian state for the next two centuries.

He is canonized in 1671 by Pope Clement X and, in Spanish, he is known as Fernando el Santo, San Fernando or San Fernando Rey.