Abstract: Streptococcus agalactiae, also known as group B Streptococuus (GBS), is an opportunistic Gram-positive bacterium with a broad host range that including humans, terrestrial animals, and aquatic species. China has become the world's largest producer and consumer of tilapia. However, S. agalactiae infections severely impede the development of the industry. To assess the virluence and immune efficacy of two mutant strains (ΔcrRNA and ΔcpsE) derived from the piscine S. agalactiae strain GD201008-001, and to develop novel strategies for preventing and controlling S. agalactiae infections in tilapia in China, we first analyzed the epidemiological profiles of S. agalactiae isolates from tilapia in China. Based on molecular epidemiological analysis of 49 publicly available S. agalactiae genomes from tilapia, we found that the dominant isolates in China belong to serotype Ia and sequence typing (ST) 7, which clustered within a distinct phylogenetic clade. Safety analysis revealed that, compared to the wild-type strain GD201008-001 (LD₅₀ = 1.04×10⁶ CFU), both the ΔcrRNA (LD₅₀ = 1.36×10⁸ CFU) and ΔcpsE (LD₅₀ > 1.0×10⁹ CFU) strains exhibited significantly attenuated virulence in Nile tilapia (Oreochromis niloticus), with no observable histopathological lesion in brain or spleen tissues. Efficacy analysis demonstrated that the ΔcrRNA strain provided higher RPS than ΔcpsE strain via both injection or immersion vaccination, with 100% RPS following intraperitoneal administration and 78.57% RPS following immersion immunization within 14 dpi. Furthermore, ΔcrRNA strain induced a significant upregulation of immune-related genes (MHC Ⅱβ, CD4-1, IFN-γ and IgM). This study systematically evaluates and compares the immunological efficacy of the ΔcrRNA and ΔcpsE strains, providing a new vaccine candidate for the prevention and control of S. agalactiae infections in Oreochromis niloticus.