The annual reproductive cycle of the Indo-Pacific tellinid clam Heteromacoma irus (Hanley, 1845) was investigated, using histology, for the first time in this study. The sex ratio of H. irus collected from a rocky intertidal area on the northern coast of Jeju Island, Korea, was found to be 1:1, indicating that H. irus is gonochoric. In January, most clams were in the resting phase, although a small number of females showed small oogonia on their follicular epithelium. From February to May, oocyte diameter increased rapidly, and fully mature eggs (44.8–56.8 μm) appeared in May. The first spawning males and females were observed in July, when the water temperature reached 18.0°C, and spawning activity continued until the end of August. During September and December, most clams were sexually in the resting stage. H. irus is considered to be a summer spawner, and annual gametogenesis was closely associated with the seasonal variation of the surface water temperature. Despite warm water temperatures in Jeju, the resting phase of female and male clams at the study site was prolonged from October to March, suggesting that food availability at this site could be poor and may act as a limiting factor in gametogenesis.