Polymer nanocomposites reinforced with inorganic fillers have sparked new aerospace, sports goods, automotive, and civil engineering applications. Here, epoxy nanocomposites with both hydrophobic and hydrophilic silica aerogel powder fillers are presented. The use of a high porosity, mesoporous filler such as silica aerogel avoids the typical problems encountered in dispersing nanoparticles. For both types of aerogel surface chemistry, the addition of minor amounts of silica aerogel leads to a strong increase of application relevant properties, e.g., fracture toughness and energy, impact strength, T g, and storage modulus. The strong covalent silica–epoxy interactions seen for the hydrophilic filler, but absent for the hydrophobic filler, are reflected in the bulk properties. Detailed fractography reveals three active toughening mechanisms: (i) an increase in nanoscale fracture roughness, (ii) crack front bowing and deflection, and (iii) the formation of shear bands. The industrial availability of silica aerogel powders, the excellent properties, and the ease of preparation of the epoxy composites make silica aerogels exceptional nanoporous fillers for polymer reinforcement.