Niceforo's Wren (Thryothorus nicefori) is a critically endangered songbird endemic to dry forests of the Chicamocha Valley in central Colombia. Discovered in 1946 and "red-listed" by the IUCN since 1988, Niceforo's Wren faces a threat of extinction because of its limited habitat range and increasing deforestation for agriculture and livestock. This species has been poorly studied, and its vocal behavior is yet undescribed. We provide the first description of the vocalizations of Niceforo's Wren, based on analyses of recordings from 29 individuals (more than half of all estimated living individuals of this species). We examined the fine structure of their songs, the pattern of solo and duet singing behavior, and the song repertoire size of males and females. Both sexes produce solo songs, coordinated vocal duets, and a variety of calls. Songs have a stereotyped syntax including one or more introductory syllables, a trill of rapidly repeated syllables, and a frequency-modulated terminal syllable. Males have a repertoire of at least 12–21 different song types, whereas females have a repertoire of at least 7–9. Both sexes sing solo songs with eventual variety. Male songs are more complex and have lower frequency characteristics, whereas female songs are shorter, with fewer syllable types and fewer syllable repeats. Duets involve mated pairs singing in a coordinated fashion using the same song types from their solo repertoires. Duets typically begin with a male song followed by a female song and show variable levels of complexity involving one to two songs given by each individual. Our results provide a foundation for monitoring Niceforo's Wrens and facilitating conservation of this critically endangered species.