A nanocomposite of polyluminol (poly(5-amino-2,3-dihydro-1,4-phthalazinedione)) and reduced graphene oxide nanosheets was electrochemically synthesized, characterized by scanning electron microscopy, X-ray diffraction and electrochemical methods, and its pseudocapacitance behavior was investigated. The excellent supercapacitive properties of the nanocomposite arising from both the electrical double layer capacitance of graphene nanosheets and pseudocapacitance of polyluminol resulted from a synergistic effect of the complementary properties of graphene and polyluminol. The nanocomposite presented a specific capacitance of 218.7Fg−1 at a discharge current of 0.55Ag−1. The nanocomposite was kept >99% of the initial capacitance after 1000 charge/discharge cycles.