Streptococcus has emerged as an important pathogen in fish farming worldwide. In this study, we characterized a Streptococcus species from a cultured, diseased Siganus guttatus in South China. Phenotypic, chemotaxonomic, 16S rRNA gene sequencing, phylogenetic and genome comparison analysis indicated that this isolate may be considered as same species with previously reported Streptococcus sp. X13SY08 from Murray cod (Maccullochella peelii peelii). We further predicted the virulence factor using whole genome sequence and analyzed the pathogenesis of this species by challenge tests. A total of 30 virulence factors were identified on the genome. Challenge tests of S. guttatus via intrathoracic injection of strain X16XC17 exhibited 100% cumulative mortality rates at 10⁸ colony forming units (CFU) per fish, and 45% to 85% at 10⁴ to 10⁷ CFU per fish. The histopathological results showed that the spleen, kidney, liver and heart all had pathological manifestations. Additionally, a specific PCR assay was developed to detect this species. The antibiotic resistance results showed that this species was only resistant to trimethoprim- sulfamethoxazole. This study describes a new species of genus Streptococcus as potential causative agent of diseased S. guttatus, and offers insights in the subsequent study of the virulence and pathogenesis associated with this pathogen.