As a novel kind of two-dimensional soft nanomaterial, graphitic carbon nitride (g-C3N4) has been boomingly explored in diverse fields such as photocatalysis and heterogeneous chemical catalysis, however, its potential application in proton exchange membranes (PEMs) fuel cells remains disclosed. In this study, nanocomposite membranes with varying g-C3N4 nanosheets content are fabricated by incorporating g-C3N4 nanosheets into sulfonated poly(ether ether ketone) (SPEEK). An increase in proton conductivity from 0.0606 S cm-1 of the SPEEK control membrane to 0.0786 S cm-1 of the nanocomposite membrane is achieved at the g-C3N4 nanosheets content of 0.5 wt% at 20 °C, stemming from that the improved connectivity of the ionic groups renders the acid-base pair effect and facilitates the Grotthuss-type transport of proton. Methanol permeability at room temperature decreases with the increase of the g-C3N4 nanosheets content, stemming from that the periodic vacancies in the lattice of g-C3N4 render the molecular sieving effect and the consequent high resistance for methanol permeation. An increase up to 39% in maximum power density is obtained, indicating the potential of g-C3N4 nanosheets for fuel cell application. Moreover, the superior mechanical properties of the g-C3N4 nanosheets lead to a 68% increase in ultimate tensile strength of the nanocomposite membranes (54.31 MPa at the g-C3N4 nanosheets content of 0.5 wt%).