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.