Aims Vibrio vulnificus is a common bacterial pathogen causing haemorrhagic septicaemia in eel farming. This study investigates the immunogenicity of an outer membrane protein U (OmpU) of V. vulnificus and the feasibility of the protein as a new subunit vaccine against V. vulnificus. Methods and Results Partial gene sequence of the OmpU of V. vulnificus was cloned, and then the OmpU was expressed and purified. Three groups of Japanese eels (Anguilla japonica) were intraperitoneally (i.p) injected with bovine serum albumin (BSA group), formalin-killed whole cell of V. vulnificus (FKC group) or the expressed OmpU of V. vulnificus (OMP group). On 14, 21, 28 and 42 days postimmunization (dpi), the whole blood cells were collected to evaluate the stimulation index (SI) and bactericidal activity. The serum was obtained to assess the titres of specific antibody, lysozyme activity, complement activity and bactericidal activity. The lysozyme activities in the suspension of kidney, skin mucus and liver in eels were also ascertained. The results showed that the SI and the titres of anti-V. vulnificus antibody in the OMP group was significantly increased on 28 dpi; lysozyme activity in the kidney and skin mucus of OMP group on 42 and 14 dpi were both significantly higher than BSA group; eels in OMP group showed strong bactericidal capacity on 21 and 28 days; and the relative percent survival of OMP vs BSA group after challenged by V. vulnificus on 28 dpi was 80%. Conclusions These results showed that the expressed OmpU of V. vulnificus could significantly improve the immune function of Japanese eel and the resistance of eels to the infection of V. vulnificus. Significance and Impact of the Study This study offered an alternative preliminary strategy of making aquaculture vaccines against V. vulnificus for eel farming.