In Japan three freshwater Limnomedusae have been reported: Craspedacusta sowerbii, C. iseana, and Astrohydra japonica. The latter two species, though known only from Japan, have not been reported in ninety and thirty years, respectively. The type material for C. iseana is lost, and the only known specimens of A. japonica have recently been deposited in the National Museum of Natural History, Smithsonian Institution, Washington, D.C. Craspedacusta sowerbii, on the other hand, is a cosmopolitan species found throughout the archipelago of Japan. Reports of C. sowerbii in China, Europe, and North America are commonplace in international publications, but particulars about the occurrence of this freshwater jellyfish, known as mamizu kurage in Japan, were previously not well understood by the international scientific community due to a lack of related English-language publications. The aim of this paper is to provide naturalists and citizen scientists in Japan with a means to both locate and identify mamizu kurage in all nine geographical regions of Japan. Here we provide an extensive history of the occurrence of mamizu kurage in Japan from the time of its first sighting in 1928 until 2011, while also providing personal observations on populations of C. sowerbii in Okinawa, Shiga, and Shizuoka Prefectures. The distribution list and map herein represent the most extensive compilation of data on the 217 reports of mamizu kurage in Japan, and should serve as the basis for future molecular genetics studies on its phylogeography to determine if all accounts correspond to a single species called C. sowerbii.