The genus Acromastigum is well represented in New Caledonia with 15 species and varieties known to date. We report, describe and illustrate here a new species from this French Overseas Territory. It is characterized by 1) transversely inserted, longitudinally plicate in a right angle, asymmetrical leaves, the dorsal leaf lobes being smaller than the ventral ones, 2) leaf cells thick-walled with inconspicuous trigones, 3) asymmetrical trifid underleaves and 4) stems with seven ranks of enlarged thick-walled cortical cells and five of thin-walled medullary cells. We discuss its belonging to the section Subcomplicata of the genus and provide the main distinctive features in relation to locally known species.