An ITO substrate with periodic surface nanostructures was used to induce strong diffusion and diffraction of incident light. The nanostructures were fabricated using nanoimprint lithography on photoresist followed by coating of the ITO layer and organic materials with uniform morphology. The nanostructures embedded into the ITO layer were found to increase absorption in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester solar devices. The short-circuit current of the nanostructured organic solar cells improved from 7.07 to 10.76 mA/cm2. This improvement was due to the increased effective optical path of absorbed light resulting from the trapping and scattering by the nanostructures.