Highly porous poly(dimethyl siloxane) (PDMS) composites containing cellular-structured microscale graphene foams (GFs) and conductive nanoscale carbon nanotubes (CNTs) are fabricated. The unique three-dimensional, multi-scale hybrid composites with inherent percolation and a high porosity of 90.8% presents a remarkable electromagnetic interference shielding effectiveness (EMI SE) of ∼75 dB, a 200% enhancement against 25 dB of the composites made from GFs alone with the same graphene content and porosity. The corresponding specific EMI SE measured against the composite density is 833 dB cm3/g. These values are among the highest for all carbon filler/polymer composites reported thus far. Significant synergy arises from the hybrid reinforcement structure of the composites: the GFs drive the incident microwaves to be attenuated by dissipation of the currents induced by electromagnetic fields, while the CNTs greatly enhance the dissipation of surface currents by expanding the conductive networks and introducing numerous interfaces with the matrix.