There are provided an image processing apparatus, an endoscope system, and an image processing method for assisting diagnosis more effectively by calculating more intuitive and useful information than blood vessel information for a blood vessel having a specific thickness.An image processing apparatus includes an image acquisition unit that acquires an endoscope image, a blood vessel extraction unit that extracts a blood vessel having a specific thickness from the endoscope image, a blood vessel information calculation unit that calculates a plurality of pieces of blood vessel information regarding the blood vessel extracted by the blood vessel extraction unit, and a blood vessel parameter calculation unit that calculates a blood vessel parameter, which is relevant to the blood vessel having the specific thickness, by calculation using the blood vessel information.
