Polymer nanocomposites based on polyaniline (PANi) and graphene nanosheets (GNS) modified with poly(sodium 4-styrensulfonate) (PSS-GNS) were prepared, and their structure and properties were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), UV-vis spectroscopy, ATR-IR spectroscopy, X-ray diffraction, elemental analysis, thermogravimetric analysis (TGA) and electrical conductivity measurements. The results revealed that for the PANi/PSS-GNS nanocomposites, the disordered structure of PSS-GNS was fully destroyed and PSS-GNS exists in the form of a single GNS or stacked PSS-GNS elements in a PANi matrix. PSS-GNS was partly covered by PANi due to hydrogen bonding that occurs between the PSS-GNS and PANi. By incorporating PSS-GNS, the electrical conductivity of PANi increased linearly from 0.84 S/cm for neat PANi to 4.96 S/cm for a PANi/PSS-GNS (5%) nanocomposite. The thermal stability of the PANi was also improved significantly to approximately $100^{circ}C$ by the nanocomposite.