A new UV-A photodetector based on K2Nb8O21 nanowire is successfully fabricated for the first time. The potassium niobate is synthesized using a facile molten method. The K2Nb8O21 nanowire photodetectors exhibit an excellent sensitivity and wavelength selectivity with respect to UV-A light. Furthermore, the photodetectors show great advantages in response time compared with other sensors based on single-oxide semiconductor nanostructures, and, especially, the responsivity is much better than that of single ZnS nanobelt photodetectors. The mechanism of conductivity is explained from the viewpoints of field emission and thermionic field emission for the change of light intensities.