Documenting the breeding habitat and distribution of migratory songbirds is essential for accurately assessing their conservation status. The “sagebrush” subspecies of the Brewer’s Sparrow (Spizella breweri breweri) breeds in greatest abundance in sagebrush-dominated (Artemisia spp.) shrublands of western North America, and the “timberline” subspecies (S. b. taverneri) breeds in shrubs or krummholz at or above treeline in mountain ranges from Alaska to Montana. Brewer’s Sparrows have also been reported at alpine sites in summer in mountain ranges across the western United States, but their taxonomic affiliation and breeding status are unknown. We reviewed monitoring, citizen science, and specimen data for Colorado and identified 186 historical summer observations of Brewer’s Sparrows at 59 alpine sites (3334–4288 m elevation). We surveyed 39 alpine sites in June-July 2021–2023, detected a total of 100 adults (mostly singing males) at 26 alpine sites (3395–3754 m elevation), and confirmed breeding at three sites. Males occupied mixed-species willow patches 0.9–1.8 m tall, often intermixed with sparse conifer krummholz. We recorded songs and captured, measured, photographed, and collected blood and feather samples from birds at a subset of alpine sites and nearby, lower-elevation sagebrush sites in May-July 2021. Vegetation associations and timing of breeding at alpine sites closely matched those of taverneri, but short songs, external morphology, coloration, and genetics of alpine birds overlapped with sagebrush birds and more closely matched those of range-wide breweri. Our results indicate that western Colorado supports a widely-distributed, but poorly-documented population of alpine breweri breeding in willows with sparse conifer krummholz within a relatively narrow elevation band at or above treeline. Whether alpine birds are itinerant breeders that first nested in sagebrush remains unknown. Our results complicate interpretation of differences in breeding habitat and breeding phenology as supporting criteria for subspecific identification and taxonomic delineation in this species.