San Ildefonso is a city located in the Ilocos region of the Philippines. It is situated within the northern part of Luzon and forms part of the Ilocos administrative region.
The city is named after Saint Ildefonso, following the Spanish colonial naming tradition common throughout the Philippines. Like other municipalities in the Ilocos region, San Ildefonso has historical ties to Spanish colonial settlement and development, though specific details about its particular history and notable features remain limited in widely available sources.