Provided are an image processing device and an electronic endoscope system that can acquire a high-quality endoscope observation image by performing sufficient noise reduction processing even if an image capture environment changes. A noise reduction processing unit sets a pixel of interest and a neighboring pixel out of a plurality of pixels that constitute an endoscope observation image, and performs noise reduction processing on the pixel of interest according to a magnitude relationship between: a difference in pixel value between the pixel of interest and the neighboring pixel； and a reference threshold value. A reference threshold value changing processing unit changes the reference threshold value for noise reduction processing based on a ratio between at least two color components that are included in the pixel of interest.
