Seahorses ( spp.) are an iconic and ecologically important species worldwide. Within the past decade, overfishing and habitat destruction has drastically reduced wild seahorse populations. Seahorse aquaculture was thus developed as an important strategy to alleviate pressure on wild stock; however, these farms have been severely afflicted with tail rot, a serious disease in seahorses. To identify the agent causing tail-rot disease in juvenile lined seahorses (), this study characterized the pathogen's physicochemical properties, extracellular product (ECP) activity, and drug susceptibility. One bacterial strain with superior growth (HM-10) was isolated from cultured liver samples of diseased juvenile lined seahorses. Healthy juvenile lined seahorses were then challenged with the strain through intraperitoneal injection. The results confirmed that HM-10 was pathogenic, causing tail-rot symptoms in challenged seahorses. The median lethal dose ( cfu/g body weight. Results from the API 20E identification system showed that under L-arabinose and amygdalin-derived acid production, arginine dihydrolase, lysine decarboxylase, and ornithine decarboxylase activity in HM-10 were negative, positive, and positive, respectively. Morphological observation, biochemical characteristics, and genetic analysis identified HM-10 as . The genetic analysis involved sequence detection of housekeeping gene 16S rRNA, including DNA gyrase subunit B gene (), DNA recombination protein gene (gyrB--recA. The ECPs of HM-10 were extracted and their properties analyzed. The results showed that the ECPs had clear hemolytic activity in response to sheep red blood cells, while also exhibiting protease, gelatinase, urease, amylase, and hemolytic activity, but not lipase or lecithin activity. Susceptibility experiments involving 33 drugs indicated that HM-10 was highly sensitive to 17:cefepime, ceftriaxone, cefotaxime, ceftazidime, cefepime, cefepime, cefoxitin, cefoperazone, kanamycin, spectinomycin, tobramycin, piperacillin, ampicillin, streptomycin, nitrofurantoin, chloramphenicol, and polymyxin B. In conclusion, the HM-10 strain of was the bacterial pathogen causing tail rot in lined seahorses. Our results are useful as a reference for the prevention and control of vibriosis in cultured seahorses.