Madrynomya bruneti n. gen. and sp. is an undescribed bivalve collected from the late Miocene Puerto Madryn Formation in northern Patagonia. The specimens were collected together with other generally well-preserved mollusks from a parauthochtonous assemblage within a fine-grained sandstone. Based on their unique shell morphology, the specimens are placed in the Modiomorphoidea, a group of bivalves known only from the Paleozoic and Mesozoic. This conclusion is based on comparisons with other bivalve taxa such as Mytiloida, Dreissenidae, Modiomorphidae, Hippopodidae, and Anomalodesmata, which they resemble in different morphological features. However, careful inspection of key characters suggests that none of them are closely allied to this new genus, and placement in the Modiomorphidae is only tentative until more material is found and further characters, such as shell microstructure, can be properly analyzed. Likewise, an accurate reconstruction of this bivalve's life habit is presently impossible to determine with the two available specimens, but an endobyssate mode is probable, as has been inferred for Myoconcha.