Streptococcus iniae has become the most serious bacterial pathogen in the aquaculture of Tachysurus fulvidraco in China. In this study, we selected S. iniae strain ZQ2205 isolated from T. fulvidraco to elucidate the biological characteristics and whole-genome information of S. iniae, thereby providing theoretical support for disease diagnosis and prevention. The growth characteristics, pathogenicity, whole-genome sequencing, and gene composition were analyzed. The results indicated that strain ZQ2205 is a gram-positive bacterium, arranged in single chains, exhibiting β-hemolytic activity. The strain demonstrated lipase, amylase, and protease activities but lacked cellulase activity. It demonstrated a wide range of tolerance to pH and salinity, being able to grow in brain heart infusion (BHI) broth at pH values ranging from 7 to 9 and salinity from 0 to 40. Strain ZQ2205 showed optimal growth at a pH of 9 and salinity between 0 and 20. The antibiotic sensitivity test showed that ZQ2205 is resistant to enrofloxacin and streptomycin, while its response to ofloxacin, tetracycline, chloramphenicol and bactrim is intermediate. Infection experiments revealed that the challenge dose as low as 0.5 CFU/g caused 100% mortality in T. fulvidraco juveniles, causing significant damage to the liver, spleen, kidneys, gills, and brain tissues, with the highest bacterial loads observed in the spleen and kidneys. Whole-genome sequencing showed that the whole genome size of ZQ2205 was 2128105 bp with a GC content of 36.83%, encoding 2035 genes. Eighteen virulence factor genes were identified, primarily involved in immune modulation, adherence, exo enzyme, and nutritional and metabolic factors. Additionally, 21 antibiotic resistance genes and 18 antibiotic target genes were predicted, mainly associated with penicillins, daptomycin, rifamycins, sulfonamides, and quinolones. In summary, these findings demonstrated that S. iniae strain ZQ2205 possesses high pathogenicity and multidrug resistance and robust environmental adaptability. The whole genome information revealed characteristics of virulence and antibiotic resistance, which are of significant importance for preventing and controlling this disease.