Introducing new sorbents is an interesting and debatable issue in the field of sample preparation. In this study, for the first time, a pH-sensitive magnetic nanoparticles microgel, Fe₃O₄–SiO₂–oly(4-vinylpyridine), was introduced as a new sorbent. The operating mechanism of this sorbent is based on changing the pH value of the sample and consequently the structure of this pH-sensitive microgel is changed. So that, at pH 6.0 the microgel was ready to accept and load the analytes (partial swelling), and when the pH was increased to 8.0, the microgel was closed and analytes were trapped inside the sorbent (deswelling). At pH 2.0 the microgel was opened and the analytes were released from the microgel (swelling). As the adsorption and desorption mechanism is based on changing the pH and only aqueous medium is used as the effluent solvent, this method is introduced as a green extraction method. The use of this microgel resulted in excellent figures of merit. The limits of quantitation and detection for herbicides were obtained within the range of 10–30 and 3–10 ng mL⁻¹, respectively. Finally, the proposed method was successfully applied to determine the concentration of phenoxy acid herbicides as hazardous materials in water samples.