The bivalve Tellimya fujitaniana (Yokoyama, 1927) (Galeommatoidea, Heterodonta) was described based on a fossil shell. Until now, the biology of living animals has not been reported. In this study, we found T. fujitaniana in a commensal relationship with the heart urchin Echinocardium cordatum (Pennant, 1777) (Spatangoida, Echinoidea) on the intertidal mud flats of the Seto Inland Sea, Japan. We investigated the morphology, host associations, and reproductive biology of this bivalve species. The elongate-ovate shell is covered by a reddish-brown ferruginous deposit. The mantle is exposed anteriorly to form a temporal siphon, while posteriorly one pair of short tentacles is exposed. Small individuals (shell length, SL, ≤ 2.1 mm) were attached to the host's body surface; middle-sized individuals (SL 3.0–3.2 mm) were attached to or stayed close to larger T. fujitanianathat were living freely in the host burrow. Nearly all the large individuals (SL ≥ 4.8 mm) lived freely in the host burrow, behind the urchin. This suggests that the host utilization pattern of T. fujitanianachanges with development. Specimens with SL ≥ 4.8 mm had mature gonads, mostly occupied by ova, and some individuals were brooding eggs or veliger larvae in the gills. This species was previously assigned to Fronsella. However, the morphology and ecology of this bivalve are very similar to those of Tellimya ferruginosa (the type species of the genus Tellimya) in the northeastern Atlantic Ocean. Thus, we have reassigned this species to genus Tellimya. We also confirmed that T. fujitaniana and T. ferruginosa can be genetically distinguished using the mitochondrial COI gene.