There are provided an endoscope system, a processor device, an operation method, and a distance measurement device for accurately measuring the observation distance even if a colorant or residues are present in a subject. An endoscope system 10 includes a light source device 14, an endoscope 12, and an observation distance measurement unit 63. The light source device 14 emits signal light, which has a wavelength band absorbed by hemoglobin contained in the subject, to the subject. The endoscope 12 has an image sensor 48 that images the subject with reflected light of the signal light and outputs an image signal. The observation distance measurement unit 63 measures the observation distance based on the image signal.