In this study, we deposit tungsten-doped indium oxide (InWO) thin films as an active layer for ultraviolet (UV) photodetectors (PDs) by radio-frequency (RF) magnetron cosputtering and discuss the optical and electrical properties of the thin film under different process conditions. The introduction of oxygen gas during the thermal annealing of oxide thin films leads to further rearrangement of the lattice and fulfills the oxygen deficiency region. When InWO thin films are thermally annealed in oxygen ambient, its photo–dark current ratio can be greatly improved. Thus, InWO UV PDs with active layers fabricated by cosputtering with sputtering powers of 80 W for the In2O3 target and 5 W for the WO3 target followed by thermal annealing for 1 h in oxygen ambient have the highest performance with an ON/OFF current ratio greater than 106, a responsivity of 160 A W−1, and a UV-to-visible rejection ratio of 4.71 × 104. Also, time-dependent switching properties have been demonstrated.