In this paper, an N-phenylthiourea-functionalized graphene oxide (GO-NPT) composite was prepared by a condensation reaction of graphene oxide (GO) and N-phenylthiourea. This composite was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, ultraviolet-visible spectroscopy, thermogravimetric analysis, the Brunauer-Emmett-Teller method, and zeta potential measurements. By exploring the effects of pH, time, temperature, salt ion solubility, and initial methyl orange (MO) concentration on adsorption, we have found the optimal conditions for adsorption. In addition, the adsorption mechanism was explored by adsorption kinetics, adsorption isotherms, and adsorption thermodynamics. After the tests, we found that GO-NPT has excellent reusability. Compared to the parent material GO (adsorption capacity of 63.84 mg/g), the adsorption capacity of GO-NPT for MO was 141.467 mg/g. Therefore, we believe that GO-NPT is a very promising adsorbent.