Aims Antibiotics and several other chemicals have been used to prevent fish diseases. However, this situation results in economic loss for the companies in the aquaculture industry and most importantly it pollutes the environment. Cinnamic acid is a naturally occurring aromatic acid and is considered to be safe for human consumption. Therefore, in this study, the antibacterial activity of trans-cinnamic acid and commonly used antibiotics, namely chloramphenicol, vancomycin, streptomycin and erythromycin, were tested against 32 bacteria, including fish pathogens, nonpathogenic isolates and collection strains. Methods and Results Trans-cinnamic acid was applied against the bacteria using the disc diffusion and microdilution method under in vitro conditions. Antibiotics were also tested under similar conditions against all tested bacteria using the disc diffusion method. The results show that among 32 bacterial strains trans-cinnamic acid exhibited potent inhibitory effect on the Gram-negative fish pathogen Aeromonas sobria. In addition, a moderate inhibition of trans-cinnamic acid of fish pathogens Aeromonas salmonicida, Vibrio (Listonella) anguillarum, Vibrio crassostreae and Yersinia ruckeri was also observed for trans-cinnamic acid in our study. On the contrary, the majority of nonpathogenic intestinal isolates were resistant to trans-cinnamic acid. Conclusions To the best of our knowledge, this is the first report on the antimicrobial activity of trans-cinnamic acid on 24 of the studied bacteria isolated from fish. In conclusion, trans-cinnamic acid can be used as an environmentally friendly alternative additive to prevent and control primarily A. sobria, as well as other pathogenic bacteria such as A. salmonicida, V. anguillarum, V. crassostreae and Y. ruckeri. Significance and Impact of the Study This study indicated that trans-cinnamic acid may present an environmentally friendly alternative therapeutic agent against A. sobria infections in the aquaculture industry.