<p>Artificial seawater has been utilised as an alternative culture medium in the larval rearing of various marine organisms. However, the viability and the effects of artificial seawater on the early-life stages of echinoderms have not been extensively studied. In this study, we examined the impacts of artificial seawater on the larval development, physiology, metamorphosis, and settlement of the purple sea urchin <i>Heliocidaris crassispina</i>, which is an ecologically and economically important seafood native in Hong Kong waters. Our results showed that larvae cultured in artificial seawater exhibited a 31% reduction in growth rates with more than 60% increase in respiration rates and more than 80% increase in frequency of larval deformities. Larval settlement was not delayed but only 0.5% settled. Larval mortality was significantly high in artificial seawater with less than 1% compared to 14% survival in natural seawater. The differences in salt composition and carbonate chemistry between artificial and natural seawater may have detrimental effects on the sensitive developmental stages of <i>H. crassispina</i>. Nevertheless, given the increasing pollution risks affecting natural seawater quality and reliability, continued research into developing optimised artificial seawater formulations with improved ecological compatibility remains a crucial avenue for sustainable <i>H. crassispina</i> seedling production.</p>