We report the growth of GaAs nanowires (NWs) on a Ge substrate using selective-area metal-organic vapor phase epitaxy (SA-MOVPE) for solar cell applications. Vertical GaAs NWs were aligned on a non-polar Ge(1 1 1) substrate by implementing As surface treatment and buffer layer growth. The transmission electron microscope (TEM) showed the occurrence of a twin in the NW and no dislocation at the GaAs/Ge interface. Photoluminescence spectra suggested luminescence from a type-II quantum well structure originated from the twin and defect-levels due to Ga vacancies and Ge interdiffusion from the Ge substrate.