Aims This study aimed to evaluate in vitro efficacy of essential oils (EOs) and their compounds (EOCs) alone or in combination against Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis in salmonid fish. Methods and Results Antimicrobial activity of 13 EOs and 16 EOCs was investigated for four A. salmonicida subsp. salmonicida strains using broth microdilution. The checkerboard assay was used to evaluate a putative synergy between the most efficient EOs and EOCs against the tested strains. Cinnamon bark, oregano, clove, and thyme oils and their major compounds cinnamaldehyde, eugenol, carvacrol and thymol showed the lower minimum inhibitory concentration and minimum bactericidal concentration values. The association of cinnamaldehyde and eugenol (V/V: 30%/70%) showed a synergistic activity against three tested strains. The combinations of cinnamon with oregano, clove or thyme EOs showed a neutral or additive activity against all the tested strains. Conclusions Cinnamon, oregano, clove and thyme oils and their major phytochemical compounds showed strong activities against A. salmonicida subsp. salmonicida strains. Significance and Impact of the Study To reduce the use of antibiotics in aquaculture, phytochemicals such as cinnamaldehyde and eugenol can be tested alone or in combination in in vivo studies as functional feed alternatives.