There are provided an image processing apparatus, an endoscope system, and an image processing method for assisting diagnosis directly and effectively by determining the state of the mucous membrane of an observation target in consideration of a temporal change of the blood vessel of the observation target.An image processing apparatus (65) includes: an image acquisition unit (81) that acquires a plurality of endoscope images obtained by imaging an observation target at different times with an endoscope (12)； a blood vessel extraction unit (82) that extracts blood vessels from the plurality of endoscope images； a blood vessel index value calculation unit (83) that calculates a blood vessel index value for each of the blood vessels extracted from the endoscope images； a temporal change calculation unit (84) that calculates a temporal change of the blood vessel index value； a determination unit (85) that determines a state of a mucous membrane of the observation target using the temporal change of the blood vessel index value； and a monitor (88) that displays a determination result of the determination unit (85).
