Physcomitrium pygmaeum is an ephemeral moss described in 1871 from a single collection from Utah, currently considered conspecific with Physcomitrium pyriforme. The interpretation of the taxon has been problematic due to its rarity in the field, the elusiveness of the type material, and an extremely scattered and inconsistent collection record. Here we present a comprehensive description and assessment of the taxon following the identification of the original material and lectotype designation, the examination of all existing herbarium specimens to the best of our knowledge, the collection of fresh material in Nevada, and the molecular barcoding of the latter using four plastid and two nuclear loci. Available information, albeit scant, suggests that this member of the North American bryoflora should be considered critically endangered following IUCN criteria.