An image forming system includes an image forming portion, a stack portion, a specific image processing portion, and a setting processing portion. The image forming portion forms an image on a sheet based on image data. The stack portion stores a stack of sheets on which images have been formed by the image forming portion. The specific image processing portion causes the image forming portion to form a specific image on a specific sheet that is discharged to the stack portion before or after one or more sheets on which one or more images have been formed by the image forming portion based on the image data. The setting processing portion sets a specific image formation position on the specific sheet so that the specific image is formed at the specific image formation position.