An image processing apparatus to execute noise reduction processing on image data includes a setting unit, a determination unit, and an output unit. The setting unit sets a pixel group from among a plurality of pixel group candidates. The plurality of pixel group candidates includes at least a first pixel group having a plurality of pixels being a first number of pixels or a second pixel group having a plurality of pixels being a second number of pixels which is different from the first number of pixels. The determination unit determines, based on a similarity between a target pixel and a reference pixel that is obtained according to the set pixel group, a weight corresponding to the reference pixel. The output unit outputs a value, calculated based on a pixel value of the reference pixel and the weight, as a noise-reduced pixel value of the target pixel.