In August 2019, an unknown disease was first reported in farmed seahorses (Hippocampus erectus) in Binhai New Area in Tianjin city, China, with a cumulative mortality rate of 25% within seven days of onset. The main symptoms of the affected seahorses were discoloration of the body surface, abdominal distention, kidney erosion, and intestinal enlargement. Histopathological observation revealed damage to the diseased seahorses' intestines, liver, and kidneys. Strains HM-2019-5 and HM-2019-6 were isolated from diseased kidneys and identified as Photobacterium sp. based on physiological and biochemical tests and 16S rDNA sequencing analysis. Artificial infection experiments demonstrated that strain HM-2019-5 could cause a 90% morbidity rate in healthy seahorses. Antimicrobial susceptibility tests showed that HM-2019-5 and HM-2019-6 were resistant to 13 of 16 antimicrobial agents tested. This is the first report of photobacterial disease in a seahorse to the best of our knowledge.