Herein, we describe simple, fast and reproducible halide ion exchange reactions in CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) at room temperature. Through the simple adjustment of the halide ion concentration, the photoluminescence of these NCs can be tuned over the entire visible region (425–655 nm). Photodetector devices based on entirely inorganic CsPbI₃ NCs are demonstrated for the first time. The photodetectors exhibited a good on/off photocurrent ratio of 10⁵.