Sibbaldia procumbens (Rosaceae) is globally widespread in alpine and subalpine habitats of the Northern Hemisphere, where it thrives in areas of late-lying snow that persist into summer. Despite its widespread circumboreal distribution and local abundance, Sibbaldia is one of the rarest plants in New England, USA, occurring only in a single ravine on Mount Washington, New Hampshire. The New Hampshire population has evidently been declining for several decades, possibly the result of a combination of interspecific competition, over-collection, and changes in environmental conditions. We assessed historical trends, current status, and potential causes of decline of the New Hampshire population through evaluation of herbarium and historical records, field surveys, and photographic comparisons. Our research at seven New England herbaria revealed 236 plants with roots, including many large adult plants, on 65 herbarium sheets. Most of these plants were collected between 1846 and 1908, representing trophy collection activity characteristic of some novel species during that period, and which may have had a lasting impact on the reproductive capacity of this long-lived perennial. Contemporary surveys (within the last 40 years) establish a continuous decline in abundance over time. During timed surveys at two previously documented stations within Tuckerman Ravine and adjacent appropriate habitat, we found no evidence of the species. We observed encroachment of the local snowbank habitat by mountain alder [Alnus viridis (Chaix) DC. subsp. crispa (Aiton) Turrill], and possibly other species. Our observations support the hypothesis that a combination of over-collection, competition from robust vegetation, and local disturbance may have contributed to its decline and possible extirpation. If extirpated, Sibbaldia would be the first documented alpine vascular plant to be extirpated from New England. Because the decline of this species may have been anthropogenically facilitated, we recommend that the feasibility of re-establishment (from Mount Washington source material) be evaluated.