A novel morphology-controlled strategy has been developed to fabricate sulfonated graphene/polyaniline (SGEP) nanocomposites by liquid/liquid interfacial polymerization. Sulfonated graphene (SGE) sheets were synthesized and used as both a macromolecular acid dopant and substrate for the polymerization of polyaniline (PANI), affording the SGEP nanocomposites. The morphology of PANI in the nanocomposites can be controlled to be either nanorods or nanogranules by varying the synthesis conditions. The morphology of SGEP and the shape of PANI can be tuned by adding an additional dopant and varying the amount of SGE used, and this had a significant influence on the electrochemical performance of the nanocomposites as supercapacitor electrode materials. The SGEP nanocomposite with PANI nanorods exhibited a specific capacitance of 763 F/g with a capacity retention of 96% after 100 cycles and good rate properties. Composites obtained with HCl as an additional acid dopant with two different ratios of SGE to PANI showed higher specific capacitances of 793 and 931 F/g, but lower capacity retention after 100 cycles of 77% and 76%, respectively.