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<i>Aeromonas hydrophila</i> is a ubiquitous Gram-negative opportunistic pathogen in the freshwater environment and the most common cause of bacterial septicemia in aquaculture. In this study, we investigated the impact of carvacrol, a natural monoterpenoid found in herbs, on the virulence of <i>A. hydrophila</i> in vitro and the antibacterial effect in combination with antibiotics. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of carvacrol against <i>A. hydrophila</i> NJ-35 were 125 µg/mL and 250 µg/mL, respectively. Carvacrol could inhibit the virulence factors (biofilm, protease, exopolysaccharide, and hemolysin) of <i>A. hydrophila</i>, and the antibiofilm potential of carvacrol was further verified by microscopic detection. Transcriptional analyses showed that the gene expression of <i>flaB</i>, <i>ompA</i>, <i>aha</i>, <i>ahp</i>, <i>ela</i>, <i>act</i>, <i>aerA</i>, <i>AhyR</i>, and <i>hly</i> were marked as downregulated. The checkerboard assay results showed that carvacrol did not have an antagonistic effect in combination with antibiotics (florfenicol, enrofloxacin, thiamphenicol, or doxycycline hydrochloride) commonly used in aquaculture but possessed an additive-synergistic effect with neomycin sulfate. In vivo studies demonstrated that carvacrol protected grass carp (<i>Ctenopharyngodon idella</i>) from <i>A. hydrophila</i> infection. Our results indicated that carvacrol possessed significant anti-bacterial and anti-virulence effects on <i>A. hydrophila</i>.