A well-preserved fossil seep community has been found in a carbonate lens in the Santonian to Campanian Omagari Formation, Upper Yezo Group in the Nakagawa region, Hokkaido, north Japan. The carbonate lens (roughly ellipsoidal in plan view with a diameter of 10 m × 6 m, and a thickness of about 5 m) is composed mainly of various types of high-Mg calcite containing several to 10 mol% magnesium and little iron or manganese. The carbonate lens is divided into an upper tube worm-dominated boundstone and a lower carbonate breccia facies. In the boundstone facies, concentric cements occur in the vestimentiferan tubes, indicating that the worm tubes were conduits for seepage. Layered to veinlike precipitates of high-Mg calcite occur in the boundstone facies. The carbonate breccia facies contains clast-supported carbonate breccia with sideritic, silty and tuffaceous matrices. Chemosynthetic bivalves occur in the upper zone of the carbonate breccia. The most common of these is the lucinid Miltha sp. Others include the lucinid Thyasira sp., and vesicomyid Calyptogena. Many small molluscs occur in the matrices of the carbonate breccia. The most common of these are trochid archaeogastropods; the others are two acmaeid limpets, mesogastropods and nuculacean bivalves. Small terebratulid brachiopods are also common. The carbonate lens, with its chemosynthetic bivalves and vestimentiferan worm tubes, may have been formed by bacterial sulfate reduction and anaerobic methane oxidation, as it shows extreme ¹³C-depletion (δ¹³C = −41 to −45‰). The Omagari community resembles the modern cold-seep communities along the landward slope of the subduction-zone complex off the Pacific coast of Japan.