An image signal processor includes a noise reduction circuit, a high dynamic range (HDR) circuit and post-processing circuit. The noise reduction circuit performs noise reduction operation on a plurality of exposure noise images each respectively corresponding to one of a plurality of brightness levels to generate a plurality of exposure clean images in a first operation mode and performs the noise reduction operation on a single noise image corresponding to one brightness level to generate a single clean image in a second operation mode. The HDR circuit merges the plurality of exposure clean images to generate an HDR image in the first operation mode. The post-processing circuit generates an output image by processing the HDR image or the single clean image.