Omphalosaurus is an enigmatic marine reptile of probable ichthyosaurian affinities known from Early and Middle Triassic marine deposits of the Northern Hemisphere. Based on its unique crushing dentition combined with elongate jaws, Omphalosaurus appears to have been a specialized ammonoid feeder. Here we describe a jaw fragment from the Lower Muschelkalk of Silesia, Poland (Karchowice Formation, Anisian, Pelsonian/early Illyrian), and assign it to Omphalosaurus sp. based on tooth morphology, tooth replacement pattern, enamel surface morphology, and enamel microstructure. This is the first record of Omphalosaurus from shallow marine carbonates and from the Muschelkalk facies, a classical source of Middle Triassic marine reptiles. The discovery is unexpected because all other records of Omphalosaurus are from openwater deposits rich in ichthyosaurs and ammonoids. We reidentify an earlier putative record of Omphalosaurus from the Lower Muschelkalk of Rüdersdorf near Berlin, Germany, as the left maxilla of the common Muschelkalk placodont Placodus. This reidentification is based on shared characters of osteology, dentition, mode of tooth replacement, and tooth enamel microstructure. Data on both specimens were collected using visual inspection, micro-computed tomography (µCT), and scanning electron microscopy.