The new species Macromtrium longipapillosum D. D. Li, J. Yu, T. Cao & S. L. Guo from Japan is described and illustrated by line drawings and photographs. The new species can be distinguished from all congeners by the combination of the following features: 1) lanceolate to oblong-lanceolate leaves with obtuse, acute to broadly acuminate apices; 2) upper and median laminal cells with thin and inflated walls, with a single, large, occasionally forked papilla, 7–14 μm, occasionally up to 18 μm, 3) basal cells near costae forming a ‘cancellina region’, 4) inner perichaetial leaves ovate lanceolate, abruptly narrowed to a long acuminate upper part; 5) short and left-twisted seta; and 6) obloid capsules with well-developed exostome teeth. We also revise the circumscription of Macromitrium comatum Mitt. and discuss the relationships of M. comatum with M. longipapillosum and other relevant species.