A graphene-based magnetic nanocomposite (MNGO) was successfully synthesized using a partial reduction co-precipitation method from iron salt solution in graphene oxide (GO) and was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), and nitrogen Brunauer, Emmer, and Teller (BET) measurements. The adsorption of La (III) ions onto the MNGO from the aqueous solution was studied as batch experiments as a function of La (III) ion concentration, pH of the solution, adsorbent dosage, contact time, and temperature. The optimum pH for the adsorption of La (III) ions was shown to be 4.0 for 0.01 g sorbent dosage and the maximum adsorption of La (III) was reached within 15 min. Under the optimum experimental conditions, the adsorption efficiency was determined to be 93%. Moreover, the MNGO exhibited selectivity and interference resistance to coexisting ions for the adsorption of La(III). To determine the adsorption characteristics, models of Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, Flory-Huggins, and Brunauer, Emmer, and Teller adsorption isotherms were applied to the measurements. The Langmuir isotherm model describes the experimental data very well. The maximum adsorption capacity of MNGO was found to be 49.75 mg/g for lanthanum under Langmuir isotherm model. Moreover, a chi-squared test was used to determine the fitting degrees of the isotherms using experimental measurements.