The bacterium Aeromonas salmonicida is the causative agent of furunculosis, a systemic, ubiquitous disease of fish in the salmon family, characterized by high mortality and morbidity. Probiotics are a promising approach for prevention of furunculosis in aquaculture. A bacterial strain with anti-A. salmonicida properties, Bacillus velezensis V4, was isolated and the mechanisms underlying these properties were investigated. Anti-A. salmonicida compounds present in cell-free supernatant of V4 were purified and structurally identified as members of the iturin, macrolactin, and difficidin groups. The compounds contributed jointly to inhibition of A. salmonicida, and the diversity of the compounds was related to the versatility of their mode of action. Addition of the compounds to A. salmonicida cell suspensions reduced cell density. Analyses by confocal microscopy and scanning electron microscopy revealed cell membrane disruption, deletion of cellular content, and cell lysis of A. salmonicida. The V4 genome was sequenced, and gene clusters involved in synthesis of anti-Aeromonas compounds were detected and identified. A possible probiotic effect on growth performance of Oncorhynchus mykiss (rainbow trout) was investigated by addition of 0, 1, and 3 % (v/w) V4. Relative to control, mortality was reduced 27.25 % in the 1 % addition group and 81.86 % in the 3 % addition group. Feed coefficient ratio was reduced 19.49 % and weight gain ratio was increased 71.22 % in the 1 % addition group. Our findings demonstrate that V4 is an effective probiotic strain in O. mykiss and has clear potential for both control of furunculosis and growth promotion of aquaculture animals.