The successful induction of triploid embryos or larvae has been performed in Patinopecten yessoensis during the past 2 decades. However, no research has been reported about the performance of triploid P. yessoensis cultured in the field. This study induced triploidy in P. yessoensis by hypotonic shock and compared the growth and reproductive performance of triploids and diploids reared under commercial conditions for up to 24 mo. The main results of this study are as follows: Triploid scallops were smaller in size and weight compared with diploids and had a retarded absolute growth rate (AGR). After 24 mo of cultivation, the mean shell height, shell length, shell width, and body weight of triploids were 9%, 10%, 9%, and 25% less than diploids, respectively (P < 0.01). Although normal in sex ratio, the reproductive potential of triploids was significantly reduced. Only 87% of the triploids exhibited sex-discernible gonads during the breeding season. None of the male triploids spawned, and the percentage of female spawners among the triploid population was only 27% of that for the diploid population. The relative fecundity of triploid females was only 4% of diploid females. Triploid eggs produced mostly aneuploid larvae and had an extremely small chance of generating viable offspring when fertilized by sperm from diploid males. Most aneuploid larvae died before the D-shaped stage, and no survival exceeded 7 days. The potential to yield viable offspring from the triploid population was estimated to be only 4% × 10^-6 of that of the diploid population. Despite the growth disadvantage of triploids, this study may support, in part, the energy reallocation hypothesis, because triploid AGR was similar to diploid AGR (2% variance) during the sexual maturation season. Our results also indicate that there would be no growth advantage, but instead a disadvantage, for triploid P. yessoensis growing at the experiment site. In addition, this research provides the first evidence that viable triploid molluscs can be induced by hypotonic shock, of which the practical and evolutionary implications are also discussed.