Cyamiocardium domaneschii n. sp. is described from shallow waters off the Southeastern coast of Brazil. An anatomical study provides comparisons with other Cyamioidea. It has a small shell (up to 3.1 mm in length), valves with a rounded contour, external radial sculpture, a distinct large prodissoconch, and hinge structure characteristics of the genus Cyamiocardium Soot-Ryen, 1951. Three well-defined pallial openings are present; tentacles border the anteriormost part of the pedal opening and the inhalant and exhalant apertures. The inner demibranchs are larger than the outers, do not have interlamellar junctions, and form a spacious brooding chamber. It is dioecious and developing oocytes are covered by a thin follicular tissue, which persists as a covering of each brooded young. Offspring are retained up to the juvenile stage, and are attached to the gill filaments by a long stalk. The foot has a narrow, grooved ventral sole; no byssus threads were observed. An active burrowing or crawling behavior is suggested for C. domaneschii n. sp., as there are well developed statocysts. This finding of a cyamiid from Brazil represents the extension of the geographical distribution of this family further north in the southwestern Atlantic Ocean. This study also reveals that some anatomical characters are variable in the genus Cyamiocardium: in C. denticulatum (Smith, 1907), there are only two pallial openings and its foot does not have a ventral sole. The attachment of young to the gills are not related to nourishment, and here for C. domaneschii n. sp. it is suggested that the stalk is only used as a “safety belt”, preventing embryos and juveniles to be released before the end of their development. More studies remain to be done on the anatomy of cyamioideans, particularly on living specimens; we paid attention to common characters used for discussions on the taxonomy and biology of these bivalves.