Acarotrochus lobulatus, a new species of shallow-water calcareous benthic foraminifer, was found in core material from the meromictic Mecherchar Jellyfish Lake of Palau, northwestern equatorial Pacific. The species belongs to a new genus, Acarotrochus, which is assigned to the Family Discorbidae Ehrenberg, 1838. The principal characters of Acarotrochus are a low trochospiral test with a foramenal plate of an extension of septal flap dividing a main chamber lumen and foliar chamberlet, a deeply incised umbilical intraseptal interlocular space which is half-covered by the backward extension of the folium, and the foliar aperture opening at the peripheral end of the foliar chamber. This new species has been found only in the calcareous layer in the core investigated, where the age is estimated to be ~100 years B.P. (end of the Little Ice Age). It may have since been extirpated from the lake.