There are provided an image processing apparatus, an endoscope system, and an image processing method for calculating an index indicating a temporal change of a blood vessel of an 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； a blood vessel extraction unit (82) that extracts blood vessels of the observation target from the plurality of endoscope images； a blood vessel information calculation unit (83) that calculates a plurality of pieces of blood vessel information for each of the blood vessels extracted from the endoscope images； a blood vessel parameter calculation unit (84) that calculates a blood vessel parameter, which is relevant to the blood vessel extracted from each of the endoscope images, by calculation using the blood vessel information； and a blood vessel change index calculation unit (85) that calculates a blood vessel change index (108), which indicates a temporal change of the blood vessel, using the blood vessel parameter.
