Compared to random nanowires, vertically aligned nanowire arrays (NWAs) are especially useful as functional parts in future nanowire based devices. Here, vertically aligned ZnS NWAs have been grown on the GaAs (111)B substrates by metal–organic chemical vapor deposition. Ga was chosen as the catalyst and dopant of the ZnS nanowires during growth. The density of nanowires was controlled and their diameter was slightly tuned. The NWAs have been assembled into a photodetector by a low cost and effective method. An individual ZnS nanowire photodetector has also been fabricated for comparison. The two photodetectors are sensitive to UV but not to visible light. Under the same conditions, the photocurrent of the NWA photodetector is more stable, and five orders larger than that of the nanowire photodetector. In addition, the ZnS NWA photodetector has a fast response and an excellent repeatability, showing its potential for fast, visible-blind, UVA detection.