[Objectives] Hippocampus abdominalis is the largest of all known seahorses. To characterize the early embryonic development of H. abdominalis and provide a biological basis for captive breeding and rearing, we observed the embryonic development of this species. [Methods] We selected males with mating intervals within 0.5 h and conducted the experiment at the water temperature of 18.5 ± 0.5 ℃. Morphological observation was carried out on fertilized eggs and litters collected in the brood pouch every 30 min after the start of the experiment. The embryonic developmental status at each stage was recorded. [Results] The findings revealed that the peak mating period of H. abdominalis primarily occurred between 11:00 and 13:00, with significantly fewer or no mating behaviors observed at other time points. The eggs are oblong, measuring 2.63 to 3.37 mm in length and 1.28 to 1.75 mm in width. Upon fertilization, the yolk contracts into a spherical shape, creating a perivitelline space, with the yolk diameter shrinking to half of the egg’s long axis, while the overall size of the fertilized egg remains unchanged. The development process was categorized based on the external morphology and characteristic features of the embryos into fifteen distinct stages: cleavage stage, blastula stage, gastrula stage, neurula stage, optic vesicle and muscle ganglion formation stage, tail bud formation stage, heart formation stage, brain formation stage, ocular melanin appearance stage, dorsal fin and pectoral fin formation stage, ventricle and atrium formation stage, yolk sac circulation stage, hatching stage, pre-larval stage, and larval stage (Figs. 1, 2). Calculations indicate that after approximately 16 d and 12 h in the brood pouch, the embryos sequentially emerge. They then undergo approximately 4 days of further development within the pouch before being expelled when the yolk sac is nearly depleted, transitioning into the larval stage. The initial larvae measure between 10.58 and 13.02 mm in total length upon expulsion (Table 1). [Conclusion] This detailed developmental timeline of H. abdominalis offers valuable insights into the unique reproductive strategies and early life stages of this species, providing a foundational reference for further studies on seahorse biology, reproductive ecology, and conservation efforts. The comprehensive characterization of the development from fertilization to the larval stage underscores the complexity and specificity of embryonic development of seahorses.