In this study, a new Fe3O4/CNT@PDA hybrid magnetic nanocomposite was synthesized through a simple, environmentally friendly, cost-effective way to remove the metribuzin herbicide and was characterized by FT-IR spectroscopy, transmission electron microscopy , and Brunauer–Emmett–Teller analysis. In this research, four adsorption isotherms, namely Langmuir, Freundlich, Dubinin–Radoslovich, and Temkin were applied, and the best results were obtained with the Freundlich model indicating multilayer adsorption. The adsorption kinetics was more consistent with the pseudo-second-order kinetics. The intraparticle diffusion model proposed that the adsorption mechanism was predominantly under the control of film diffusion. The thermodynamic parameters suggested that the uptake of metribuzin onto Fe3O4/CNT@PDA was physisorption, spontaneous, and exothermic and decreased the randomness process in nature. This study showed that the Fe3O4/CNT@PDA hybrid magnetic nanocomposite can be an effective adsorbent for removing herbicides such as metribuzin in drinking water sources.