Sulfonated poly(fluorenyl ether sulfone) (PFES) with a terminally-crosslinked network structure was prepared by heat-induced crosslinking of the allyl-terminated telechelic sulfone polymers using a bisazide. The crosslinked polymer membrane with a sulfofluorenyl moiety of 70% (PFES-70) showed excellent hydrolytic, dimensional, and mechanical stability. The crosslinked PFES-70 membrane revealed a proton conductivity of 0.04 S/cm at $20^{circ}C$, which increased significantly with increasing temperature. The conductivity of 0.38 S/cm at $100^{circ}C$ for PFES-70 was higher than that of both non-crosslinked (0.33 S/cm) and $Nafion^{circledR}$ (0.17 S/cm). In addition, the crosslinked PFES-70 showed a methanol permeability of only 1.4% ($3.9{ imes}10^{-8}cm^2/s$) compared to $Nafion^{circledR}$.