Accurate segmentation of the prostate gland is a challenging task due to the high variability of prostatic anatomic structures. The diagnostic accuracy of diffusion-weighted-imaging (DWI) to detect prostate cancer (PCa) is well established. Proper delineation using DWI of the prostate gland can take an essential part in the computer-aided-diagnosis of PCa. The purpose of this study is to automatic segmentation of prostate-gland using superpixel-based segmentation in DWI and select the optimal b-value of DWI for segmentation. Experimental results provide a close match to radiologist manual delineation and able to determine the optimal b-value of DWI for superpixel-based prostate-gland segmentation.