Cellular-structured graphene foam (GF)/epoxy composites are prepared based on a three-step fabrication process involving infiltration of epoxy into the porous GF. The three-dimensional (3D) GF is grown on a Ni foam template via chemical vapor deposition. The 3D interconnected graphene network serves as fast channels for charge carriers, giving rise to a remarkable electrical conductivity of the composite, 3 S/cm, with only 0.2 wt % GF. The corresponding flexural modulus and strength increase by 53 and 38%, respectively, whereas the glass transition temperature increases by a notable 31 °C, compared to the solid neat epoxy. The GF/epoxy composites with 0.1 wt % GF also deliver an excellent fracture toughness of 1.78 MPa·m(1/2), 34 and 70% enhancements against their "porous" epoxy and solid epoxy counterparts, respectively. These observations signify the unrivalled effectiveness of 3D GF relative to 1D carbon nanotubes or 2D functionalized graphene sheets as reinforcement for polymer composites without issues of nanofiller dispersion and functionalization prior to incorporation into the polymer.