The continuous increase in atmospheric carbon dioxide concentrations is driving ocean warming and acidification, with Korean coastal waters experiencing accelerated warming and acidification, significantly threating coastal ecosystems. This study investigated the growth responses of the diatom Chaetoceros constrictus, a species that annually dominates the phytoplankton community in Busan coastal waters, under varying temperature (20, 22.5, 25°C) and pH (8.6, 7.6, 6.6, 6.1) conditions using clonal culture experiments. As temperature increased, the growth rate decreased significantly, and the time to reach maximum cell abundance was delayed, with a marked reduction in maximum cell abundance observed at 25°C. Growth rates and maximum cell abundances declined as the pH decreased, although the effects were less pronounced compared with temperature changes. These findings suggest that C. constrictus thrives best at 20°C and pH 8.6, indicating that temperature has a greater impact on its growth than pH. These results reflect the differences in physiological stress and adaptive mechanisms under temperature and pH changes, providing valuable insights into the impacts of climate change on marine ecosystems and biological carbon pumps.