The effects of CO2 enrichment on various ecophysiological parameters of tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers were tested. T. hemprichii, collected from a seagrass bed in Xincun Bay, Hainan island of Southern China, was cultured at 4 CO2(aq) concentrations in flow-through seawater aquaria bubbled with CO2. CO2 enrichment considerably enhanced the relative maximum electron transport rate (RETRmax) and minimum saturating irradiance (Ek) of T. hemprichii. Leaf growth rate of CO2-enriched plants was significantly higher than that in unenriched treatment. Nonstructural carbohydrates (NSC) of T. hemprichii, especially in belowground tissues, increased strongly with elevated CO2(aq), suggesting a translocation of photosynthate from aboveground to belowground tissues. Carbon content in belowground tissues showed a similar response with NSC, while in aboveground tissues, carbon content was not affected by CO2 treatments. In contrast, with increasing CO2(aq), nitrogen content in aboveground tissues markedly decreased, but nitrogen content in belowground was nearly constant. Carbon: nitrogen ratio in both tissues were obviously enhanced by increasing CO2(aq). Thus, these results indicate that T. hemprichii may respond positively to CO2-induced acidification of the coastal ocean. Moreover, the CO2-stimulated improvement of photosynthesis and NSC content may partially offset negative effects of severe environmental disturbance such as underwater light reduction.