Covalently crosslinked graphene oxide papers (GOPs) with enhanced mechanical properties are prepared by a strategy involving crosslinking by means of intercalated polymers. The strength and modulus of the crosslinked GOPs increase by 115% and 550%, respectively, compared to the pristine GOPs. These results broaden the potential applications of graphene, and the crosslinking strategy will open the door to the assembly of other nanometer-scale materials.