An in situ emulsion polymerization method was used for the synthesis of polyacrylonitrile nanoparticles amino-functionalized partially reduced graphene oxide (PAN-PRGO). After that, hydrolyzed polyacrylonitrile nanoparticles amino-functionalized partially reduced graphene oxide (HPAN-PRGO) nanocomposite was achieved by the modification of nitrile groups of the composite polymer chains to carboxylic groups, aminoethylene diamine, and amidoxime functional groups through partial hydrolysis using a basic solution of sodium hydroxide for 20 min. Different synthesized materials were characterized and compared using well-known techniques including transmission electron microscope (TEM), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectra, and X-ray diffraction (XRD). The nanocomposite was structured through the interaction between acrylonitrile’s (AN) nitrile groups and amino-functionalized graphene oxide nanosheets’ amino groups to successfully graft polyacrylonitrile over the surface of functionalized nanosheets as approved by characterization techniques. The synthesized composite was examined for the removal of samarium ions (Sm3+) from water. Different experimental conditions including pH, contact time, initial concentration, and adsorbent dose were investigated to determine the optimum conditions for the metal capture from water. The optimum conditions were found to be a contact time of 15 min, pH 6, and 0.01 g of adsorbent dosage. The experimental results found, in a good agreement with the Langmuir isotherm model, the maximum adsorption capacity of Sm3+ uptake was equal to 357 mg/g. A regeneration and reusability study of synthesized composite up to six cycles indicated the ability to use HPAN-PRGO nanocomposite several times for Sm3+ uptake. The obtained results prove that this polymer-based composite is a promising adsorbent for water treatment that must be studied for additional pollutants removal in the future.