We report a case of probable interspecific song learning in a male Florida Grasshopper Sparrow (Ammodramus savannarum floridanus), a critically endangered grassland bird. This individual was observed to sing a distinctive song where the second phrase of the song accurately resembled a typical song of a Bachman’s Sparrow (Peucaea aestivalis), and the introductory phrase was intermediate between the songs of Bachman’s and Grasshopper sparrows. The exact mechanism of development of this unusual song is unknown, but scarcity of species-typical tutors as a result of rapid population decline may have resulted in interspecific song development. The atypical individual exhibited a strong response to recordings of conspecific song and successfully defended a territory in high-quality habitat but nevertheless appeared to be unpaired. Improper song learning may inhibit mate attraction and qualify as an Allee effect contributing to the extirpation of small populations.