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Pseudomonas plecoglossicida is a Gram-negative fish pathogen that affects the large yellow croaker (LYC; Larimichthys crocea) and several other economically important fish species. The bacterium can cause systemic infections in LYC, resulting in visceral granulomas disease (VGD) with high mortality in cultured populations. Despite the severity of this disease, the LYC industry lacks an effective vaccine for VGD. We have constructed a highly attenuated P. plecoglossicida mutant by deleting a virulence gene (tssD-1) from the wild-type (WT) strain XSDHY-P. We then investigated the safety and efficacy of the mutant strain Δ tssD-1 in LYC juveniles as a live attenuated vaccine candidate. LYCs were vaccinated with Δ tssD-1 by two intraperitoneal injections per fish, and then challenged with P. plecoglossicida XSDHY-P by the immersion route. Vaccination of Δ tssD-1 induced a significantly higher level of P. plecoglossicida -specific serum IgM and increased expression of CD8α and MHCIα, and resulted in a relative percentage survival of 86.3%. The safety of Δ tssD-1 was demonstrated because no mortality or clinical signs were found in a LYC group injected with a high dose (1.8 × 106 CFU per fish) of Δ tssD-1. This is the first report of a defined live attenuated strain providing protective immunity in LYC against P. plecoglossicida. • Attenuated Pseudomonas plecoglossicida strain Δ tssD-1 is safe and immunogenic as a fish vaccine. • Δ tssD-1 induces both humoral and cellular immune responses. • Δ tssD-1 provides protective immunity against P. plecoglossicida infection. [ABSTRACT FROM AUTHOR]