Various kinds of fumed silica nanoparticles, different in terms of specific surface area and surface functionalization, were melt compounded with a poly(lactic acid) biodegradable matrix, with the aim to investigate the thermo-mechanical and optical properties of the resulting materials. Untreated nanoparticles at elevated surface area resulted to be effective in increasing elastic modulus, because of the extended filler–matrix interaction, while the finer dispersion of silica aggregates at the nanoscale obtained with surface treated nanoparticles led to noticeable improvements of the tensile properties at yield and at break, both under quasi-static and impact conditions. Also the fracture toughness and the creep stability were substantially enhanced by nanosilica addition, without impairing the original optical transparency of the matrix.