Conspecific sperm precedence occurs when females are exposed to sperm from males of multiple species, but preferentially use sperm of a conspecific. Conspecific sperm precedence and its mechanisms have been documented widely in terrestrial species, in which complex female behaviors or reproductive tract morphologies can allow many opportunities for female choice and sperm competition, however, the opportunity for conspecific sperm precedence in free spawning marine invertebrates has been largely ignored. Two sea urchin species, Echinometra oblonga and E. sp. C, have high levels of interspecific fertilization in no-choice lab crosses, but no natural hybrids have been found. We performed competitive fertilization assays to test for conspecific sperm precedence and found that eggs of both species showed a marked preference for conspecific sperm when fertilized with heterospecific sperm mixtures. Strong rejection of heterospecific sperm would not have been predicted from no-choice assays and helps explain the lack of natural hybrids. We also found significant variation in hybridization success among crosses. Conspecific sperm precedence in free spawning invertebrates shows that the simple surfaces of eggs and sperm provide ample opportunity for egg choice and sperm competition even in the absence of intricate behavior or complex reproductive morphologies.