An image sensor may include a device isolation structure defining a plurality of pixel regions in a substrate and a photoelectric conversion element formed in each of the pixel regions. The device isolation structure may include an insulating gapfill layer extending from an upper portion to a lower portion of the device isolation structure, a spacer provided at the upper portion of the device isolation structure and interposed between the insulating gapfill layer and the substrate, and a lower impurity region provided at the lower portion of the device isolation structure and interposed between the insulating gapfill layer and the substrate.