In recent years, new emerging pathogenic microorganisms have frequently appeared in animals, including marine fish, possibly due to climate change, anthropogenic activities, and even cross-species transmission of pathogenic microorganisms among animals or between animals and humans, which poses a serious issue for preventive medicine. In this study, a bacterium was clearly characterized among 64 isolates from the gills of diseased large yellow croakerLarimichthys croceathat were raised in marine aquaculture. This strain was identified asK. kristinaeby biochemical tests with a VITEK 2.0 analysis system and16S rRNAsequencing and namedK. kristinae_LC. The potential genes that might encode virulence-factors were widely screened through sequence analysis of the whole genome ofK. kristinae_LC. Many genes involved in the two-component system and drug-resistance were also annotated. In addition, 104 unique genes inK. kristinae_LCwere identified by pan genome analysis with the genomes of this strain from five different origins (woodpecker, medical resource, environment, and marine sponge reef) and the analysis results demonstrated that their predicted functions might be associated with adaptation to living conditions such as higher salinity, complex marine biomes, and low temperature. A significant difference in genomic organization was found among theK. kristinaestrains that might be related to their hosts living in different environments. The animal regression test for this new bacterial isolate was carried out usingL. crocea, and the results showed that this bacterium could cause the death ofL. croceaand that the fish mortality was dose-dependent within 5 days post infection, indicating the pathogenicity ofK. kristinae_LCto marine fish. SinceK. kristinaehas been reported as a pathogen for humans and bovines, in our study, we revealed a new isolate ofK. kristinae_LCfrom marine fish for the first time, suggesting the potentiality of cross-species transmission among animals or from marine animals to humans, from which we would gain insight to help in future public prevention strategies for new emerging pathogens.