Ildefonsus or Ildephonsus (rarely Ildephoses or Ildefonse; Spanish: San Ildefonso; c. 8 December A.D. 607 – 23 January A.D. 667) was a scholar and theologian who served as the metropolitan Bishop of Toledo for the last decade of his life.

He relates that Ildephonsus was praying one day before the relics of Saint Leocadia when the martyr arose from her tomb and thanked him for the devotion he showed towards the Mother of God.

[4] It was reported that on 18 December 665 he experienced a vision of the Blessed Virgin when she appeared to him in person and presented him with a priestly vestment, to reward him for his zeal in honouring her.

She praised Ildefonsus for his devotion, and vested him with a special chasuble from her son's treasury, which she instructed the bishop to wear only during Marian festivals.

Even during the Muslim occupation, when the basilica was converted into a mosque, the area where the vision occurred remained sacred and dedicated to the Virgin Mary.

In Olula del Río (Almería), festivities begin with a bonfire and fireworks the previous night, and continue with a traditional procession of the saint's image with bread being tossed onto onlookers.

Ildefonsus utilises Isidore's "synonymous method" (or Synonyma Ciceronis) for theological purposes, wherein he repeats every phrase several times in different, although purportedly identical, ways.

Ildefonsus also probably wrote the Visigothic Mass of Ascension, which explains how the benefits received from Christ are richer than the wonders he performed, such as "ascending unaided to the clouds".

[9] Theologically, Ildefonsus regarded the Nicene Creed as sufficientem scientiam salutarem (sufficient knowledge for salvation) and as a foedus (compact) between believer and God.

[10] Like Isidore of Seville before him, Ildefonsus regarded the creed as forming "two pacts" between God and believer: one renouncing the devil, and another the statement of belief itself.

[16] In his De cognitione baptismi, Ildefonsus explained the biblical origins of the sacrament, as well as Hispanic baptismal practices and important prayers.

Julian of Toledo in the Elogium Ildefonsi mentions two lost works by Ildefonsus: Liber Prosopopoeia Imbecillitatis Propriae and Opusculum de proprietate personarum Patris, et Filii et Spiritus Sancti.