KNbO3, a type of perovskite material, was widely developed for various devices due to its excellent light absorption properties, non-toxicity, chemical stability, ferroelectricity, and so on. As ferroelectric material, the KNbO3 can generate a depolarized electric field for separation of photon-generated carriers. In this work, KNbO3 nanocubes were synthesized using a hydrothermal method and integrated as a photoanode material into a PEC-type UV detector. The UV detector possesses a fast response time with the corresponding rising and recovery times of 72 and 28 ms under 35 mW cm−2 irradiance at 365 nm. In addition, the UV detector presents significant visible light blindness and good linearity and is self-powered. This work indicated that the KNbO3 could be used as a new option for the active layer of the UV detector.