The present technology relates to an image processing device, an image processing method, a surgery system, and a surgical thread capable of further improving visibility of the thread. The image processing device is provided with an image obtaining unit which obtains a first image imaged under an illumination condition in which the surgical thread fluoresces and a second image imaged under an illumination condition including at least visible light as images of an operative site using the surgical thread which fluoresces, and a synthesis unit which generates a synthetic image obtained by synthesizing an area of the surgical thread of the first image and the second image. The present technology is applicable to, for example, a surgery system and the like.