San Ildefonso, officially the Municipality of San Ildefonso is a 1st class municipality in the province of Bulacan, Philippines. According to the 2020 census, it has a population of 115,713 people.

Ildefonsus or Ildephonsus was a scholar and theologian who served as the metropolitan Bishop of Toledo for the last decade of his life. His Gothic name was Hildefuns. In the Ethiopian Orthodox Tewahedo Church he is known as Dexius based on the Ge'ez translation of legends about his life.