The Santa Marta Sabrewing Campylopterus phainopeplus is listed as a “Critically Endangered” hummingbird endemic to the Sierra Nevada de Santa Marta (SNSM) in Colombia. Prior to 2022, there were only three documented sightings of the sabrewing since it was described in 1879, including only one record between 1946 and 2022. As a result, this “lost” species has long been one of the most poorly known birds in Colombia. We located a resident population of Santa Marta Sabrewing along the Guatapurí River near the Chemesquemena and Guatapurí villages in July 2022, and at its type locality, San José, in January 2023. Based on historical data and newly collected field observations, we assess the species’ status and describe aspects of its natural history and ecology. Our review indicates that the species has been frequently misidentified in the past, and that to date, documented evidence of its presence is limited to four localities, all of them restricted to the south-eastern slope of the SNSM, along the mid Guatapurí River basin. Consequently, this bird appears to represent a case of microendemism. This species is likely to remain listed as Critically Endangered until conclusive evidence suggests otherwise. Field observations indicate that the species is highly associated with watercourses, where males hold year-round territories and form leks. We obtained records of males in mid-elevation habitats (1,150–1,850 m) for 16 consecutive months between July 2022 and October 2023, suggesting that the species might not be an elevational migrant, as previously speculated. More information is needed to understand the species’ ecology so that effective conservation actions can be designed in collaboration with the indigenous communities with which the species coexists.