El Viejo is also the site of a colonial-era church, the Basilica of the Immaculate Conception of Mary in El Viejo (Basílica Inmaculada Concepción de María en El Viejo), built in the 17th century. The church was elevated to the status of national religious sanctuary on October 8, 1995, and was declared a minor basilica on February 7, 2006 by Pope John Paul II, during his second visit to Nicaragua.
The basilica possesses a baptismal font dating from 1560, and also possesses a little statue of the Virgin Mary. Tradition states that Fray Pedro de Zepeda y Ahumada, brother of Teresa of Avila, traveling to El Realejo with the image, was forced to go to El Viejo due to a storm. The people of El Viejo were so taken by it that they wished for the image to stay in their town. However, Zepeda y Ahumada set off once more for El Realejo, but a second storm forced him back to El Viejo. Thus, he decided that this was a sign of divine intervention and left the image in El Viejo.
On December 6 of every year, the silver that is part of the little statue is washed before congregants in a ceremony called the lavado de la plata.