The breeding biology of the Santa Marta Parakeet Pyrrhura viridicata is described based on the study of the population inhabiting the San Lorenzo Ridge, an Important Bird Area within the Sierra Nevada de Santa Marta, Colombia. We performed surveys during 2005–2008 and installed 48 nest boxes at four sites in May 2006. Nineteen nesting attempts were recorded: nine were made in nest boxes and ten occurred in natural cavities. Clutches of the Santa Marta Parakeet averaged 4.5 eggs per nest and up to seven eggs were observed in a single cavity. Previously undescribed breeding behaviours are reported for the species: cooperative breeding and replacement clutching. The only natural cavities used for nesting were in dead Wax Palms Ceroxylon ceriferum. This palm is one of the most common and numerous tree species in the San Lorenzo Ridge between 2300–2700 m elevation and key for the species' breeding cycle.