Early Carboniferous (latest Visean) seep deposits occur on top of the drowned Middle Devonian–Late Devonian Iberg atoll reef, Harz Mountains, Germany. These deposits include limestone with a low-diversity but high-abundance fauna of rhynchonellid brachiopods and rare solemyid bivalves, as well as microbial limestone. Rhynchonellids form dense, autochthonous shell accumulations and are generally articulated. They are closely associated with hydrocarbon-derived carbonates. The carbonates exhibit δ13C values as low as −32‰, relative to the Peedee belemnite standard, revealing that they are predominantly hydrocarbon derived. The fauna, carbonate fabrics, and isotope signatures provide unequivocal evidence for a seep origin of the Visean deposit. The occurrence of solemyid bivalves supports this interpretation as members of this family (1) are well known for their relationship with chemoautotrophic bacteria and (2) have been reported from ancient and modern seeps. Possible hydrocarbon sources are thermogenic methane derived from the volcanic base of the Iberg reef or methane from a petroleum reservoir.