Surface plasmon resonances in metallic nanoparticles are of interest for a variety of applications due to the large electromagnetic field enhancement that occurs in the vicinity of the metal surface, and the dependence of the resonance wavelength on the nanoparticle's size, shape, and local dielectric environment. Here we report an engineered enhancement of optical absorption and photocurrent in a semiconductor via the excitation of surface plasmon resonances in spherical Au nanoparticles deposited on the semiconductor surface. The enhancement in absorption within the semiconductor results in increased photocurrent response in Si pn junction diodes over wavelength ranges that correspond closely to the nanoparticle plasmon resonance wavelengths as determined by measurements of extinction spectra. These observations suggest a variety of approaches for improving the performance of devices such as photodetectors, imaging arrays, and photovoltaics.