An image forming system includes: a data conversion unit that converts first image data into second image data; and an image output unit that outputs an image obtained by reproducing the second image data. The data conversion unit includes: a determination unit which divides the first image data into a dot block each includes plural pieces of dot data, and determines whether or not an array of dot data included in each dot block corresponds to a specific array; and a replacement unit which replaces the data out of the second image data, which is located in a portion corresponding to the dot block, with first replacement data including image forming dot data with which an image is formed. The specific array includes a first array in which the dot data to be thinned out is configured as the image forming dot data, and the dot data not to be thinned out is configured as non-image forming dot data with which an image is not formed.