A series of myricetin derivatives containing amide, thioether, and 1,3,4-thiadiazole moieties were designed and synthesized, and their antiviral and antibacterial activities were assessed. The bioassays showed that all the title compounds exhibited potent in vitro antibacterial activities against Xanthomonas citri (Xac), Ralstonia solanacearum (Rs), and Xanthomonas oryzae pv. Oryzae (Xoo). In particular, the compounds 5a, 5f, 5g, 5h, 5i, and 5l, with EC50 values of 11.5⁻27.3 μg/mL, showed potent antibacterial activity against Xac that was better than the commercial bactericides Bismerthiazol (34.7 μg/mL) and Thiodiazole copper (41.1% μg/mL). Moreover, the in vivo antiviral activities against tobacco mosaic virus (TMV) of the target compounds were also tested. Among these compounds, the curative, protection, and inactivation activities of 5g were 49.9, 52.9, and 73.3%, respectively, which were better than that of the commercial antiviral Ribavirin (40.6, 51.1, and 71.1%, respectively). This study demonstrates that myricetin derivatives bearing amide, thioether, and 1,3,4-thiadiazole moieties can serve as potential alternative templates for the development of novel, highly efficient inhibitors against plant pathogenic bacteria and viruses.