<p>Owing to overfishing and habitat degradation, the population of wild seahorses has declined substantially. Aquaculture is recognized as the most efficient approach to reconcile market demands with the conservation of wild seahorse populations. However, the lack of research on basic biology, especially histological studies, has severely hindered the advancement of large-scale seahorse aquaculture. In this study, the organogenesis of the big-belly seahorse (<i>Hippocampus abdominalis</i>), the major farmed seahorse species in China, was analyzed from 1 to 75&#xa0;days after birth (DAB), which corresponds to 19 to 1,425&#xa0;day degrees (<i>D</i>°). Three main developmental stages were identified: 1) DAB 1–10 (19–190 <i>D°</i>, early planktonic stage): On the DAB 1 (19 <i>D°</i>), the mouth and anus opened, indicating that they had acquired basic food-selecting ability. On the DAB 10 (190 <i>D°</i>), the first intestinal loop appeared in the intestine to accommodate to intestinal growth and meet increasing nutritional demands. Meanwhile, the female ovary was observed for the first time. 2) DAB 10–30 (190–570 <i>D°</i>, late planktonic stage): This stage was primarily characterized by the maturation of the respiratory and urinary systems. Notably, the brood pouch was first observed on the DAB 15 (285 <i>D°</i>). 3) DAB 30–75 (570–1,425 <i>D°</i>, benthic stage): At DAB 30 (570 <i>D°</i>), the second intestinal loop appeared in the intestine by which time the intestinal tract was basically fully developed. The depth of the ocular fovea increased, which coincided with the transition of seahorses from the planktonic stage to the benthic stage. Throughout the research period (DAB1-75, 19–1,425 <i>D°</i>), the male gonads were not found. This study fills the gaps in the biological and physiological knowledge of this species, as well as provides a theoretical foundation for further insights into the growth and developmental mechanisms of the big-belly seahorse.</p>