Zirconium oxide nanoparticles (ZrO2NPs) were green synthesized for the first time using an aqueous peppermint extract as a precipitating and capping agent. Addition of the extract to Zr4+ solution was followed by calcination of the resulting precipitate at 570 and 700 °C to form ZrO2NPs570 and ZrO2NPs700, respectively. These oxides were characterized using X-ray diffraction, transmission electron microscopy, and BET surface area analysis, and used as adsorbents for cobalt ions (Co2+) in water. The effects of pH, initial Co2+ concentration, ZrO2NPs mass, and contact time on adsorption efficiency were studied. Characterization results showed formation of cubic ZrO2 with average crystallite sizes (XRD data) of 6.27 and 7.26 nm for ZrO2NPs570 and ZrO2NPs700, respectively. TEM images of the two oxides exhibited nearly spherical nanoparticles and BET surface area measurements indicated the formation of mesoporous oxides having surface areas of 94.8 and 62.4 m2/g, respectively. The results of the adsorption study confirmed that the synthesized ZrO2NPs can be efficiently used for the adsorption of Co2+ from water. The uptake of Co2+ from the treated solution is favored at pH values higher than its point of zero charge (6.0). In addition, the adsorption of Co2+ by ZrO2 follows a pseudo-second order kinetics (R2 = 1.0) and can be explained by the Langmuir adsorption isotherm (R2 = 0.973).