Actinocythereis imbeensis n. sp. is described from the early Miocene strata of the Pelotas Basin, the southernmost Brazilian marginal basin. The specimens were recovered from the base to the top of an offshore core composed of greenish-gray shale interpreted as part of a transgressive sequence. Morphological features and faunal composition indicate that the ostracods were autochthonous and lived in the photic zone.