Elmira is a medium-to-large gastropod of uncertain systematic affinity, which has so far been reported only from a presumably Eocene methane-seep deposit in Cuba. This study reports a mass occurrence of Elmira shimantoensis Kiel and Nobuhara sp. nov. from a Late Cretaceous hydrocarbon-seep deposit in Shikoku, Japan, called the Sada Limestone. Its paleoecology is reconstructed based on its mode of occurrence, carbonate petrology, and stable carbon isotope analyses. The fauna of the Sada Limestone in general is characterized by an abundance of large chemosymbiotic bivalves of the family Thyasiridae and of "Serpula'' tubes. The mass occurrence of Elmira shimantoensis was found in a lens-shaped carbonate body with a flat top and a concave base, 6.5 m in length and less than 2 m in thickness, consisting of multiple layers of shell accumulations, which were formed by shell-winnowing and the filling of a depression in slope mud. The scarcity of Elmira shimantoensis elsewhere in the Sada Limestone suggests that it formed locally from a gregarious population in the vicinity of the depression, possibly on hard ground. The matrix of the mass occurrence is rich in dolomite and ankerite, and is less depleted in C-13 (delta C-13 values of calcite: -5.3 to -2.4 parts per thousand; of dolomite: -8.3 parts per thousand) than the matrix of the enclosing thyasiridrich and tube-rich limestones. This suggests that the gastropod mass occurrence was cemented below the sulfate reduction zone and has thus undergone little anaerobic methane oxidation. Therefore, Elmira shimantoensis is reconstructed here as a bacteria grazer on a hard substrate such as exposed carbonate mounds rather than as a species that relied on chemosynthetic symbionts for nutrition.