Former collegiate church founded in 1436 on the site of the old parish church of Castellbò, located at the foot of the hill of the former castle. It is a large building with a rich and complex architectural design. It consists of a nave ending to the north in a semicircular apse and originally covered by a shell vault with lunettes, recently replaced by a wooden beamed ceiling supported by diaphragm arches. An added section at the southern end of the nave houses a strong bell gable with three openings at the upper level. Below it is the monumental doorway, with a recessed pointed arch and topped by a circular oculus. The wrought ironwork of the portal is particularly noteworthy.