Macromitrium hainanense is described and illustrated as a new species from the Hainan Province of China. This species, belonging to a gymnostomous group of Macromitrium, is most closely related to M. taiheizanense from Taiwan. It is distinguished from all congeners by the immersed and ovoid capsules with exceedingly short setae (ca. 0.2–0.4 mm long), the absence of a peristome, the rounded-quadrate to oblate, pluripapillose upper laminal cells, the elongate and sigmoid-curved, smooth basal cells and the hairy, campanulate-mitrate calyptrae.