Based upon large cerithiform gastropod specimens from the middle Miocene beds of Java, Indonesia and the Philippines, a new genus and new species, Megistocerithium magoi, is described herein. Ten fragmentary specimens from the lower and upper Miocene beds of the Philippines are tentatively referred to this species. M. magoi Kase sp. nov. is amongst the largest fossil cerithioidean gastropods (except for Turritellidae). It was a dweller in intertidal sandy mudflats, probably in close proximity to mangrove forests. No species considered to be congeneric with this species have been found in the Cenozoic. On the other hand, M. magoi Kase gen. et sp. nov. shares apertural characters with the Middle Jurassic species Eustoma tuberculosa, the type species of a genus belonging to the yet to be well-recognized family Eustomatidae. M. magoi Kase gen.et sp. nov. is possibly a relict of the Mesozoic Eustomatidae. It is assumed to have been a grazer on mangrove litter like the potamidid genera Terebralia and Telescopium in modern mangrove swamps.