Nanostructured thin plastic foils have been used to enhance the mechanism of laser-driven proton beam acceleration. In particular, the presence of a monolayer of polystyrene nanospheres on the target front side has drastically enhanced the absorption of the incident 100 TW laser beam, leading to a consequent increase in the maximum proton energy and beam charge. The cutoff energy increased by about 60% for the optimal spheres' diameter of 535 nm in comparison to the planar foil. The total number of protons with energies higher than 1 MeV was increased approximately 5 times. To our knowledge this is the first experimental demonstration of such advanced target geometry. Experimental results are interpreted and discussed by means of 2(1/2)-dimensional particle-in-cell simulations.