Abstract: To investigate the effects of curcumin on the acquisition of exogenous plasmids via conjugation in Vibrio harveyi and the underlying mechanism, we employed V. harveyi 345 as the recipient and an Escherichia coli strain harboring the shuttle plasmid pMMB207 as the donor. The changes in the efficiency of exogenous plasmid acquisition via conjugation in V. harveyi were analyzed before and after curcumin treatment at three different stages: 1) during the early-log growth of V. harveyi; 2) during the plate-mating process; and 3) during both stages. Furthermore, qRT-PCR was further employed to monitor the expression of SOS-response genes under treatments 1 and 2. Compared with the curcumin-free control, 2, 4, 8, and 10 µg·mL⁻¹ curcumin significantly reduced the conjugative efficiency by 72%, 56%, 76% and 86% under treatment 1, and by 47%, 89%, 56%, 58% under treatment 2, respectively; whereas in treatment 3, only 10 µg·mL⁻¹ curcumin exerted a significant inhibition effect, with the conjugation efficiency decreased by 21%. In treatment 1, 2 µg·mL⁻¹ curcumin down-regulateed sulA by 1.92-fold (p<0.05). In treatment 2, 2 µg·mL⁻¹ curcumin decreased recA, lexA and sulA transcripts by 1.58- to 2.08-fold (p<0.05) after 1 h of mating. The results suggest that curcumin may reduce the transfer of conjugative plasmid, most likely by attenuating the SOS response, and highlight its potential to relieve horizontal gene transfer and the potential spread of antimicrobial resistance, providing a theoretical foundation for novel antibacterial adjuvants.