An image sensor includes a first chip including a pixel region, a pad region, and an optical black region interposed between the pixel region and the pad region, and a second chip being in contact with a first surface of the first chip and including circuits for driving the first chip. The first chip includes a first substrate, a device isolation portion disposed in the first substrate and defining unit pixels, an interlayer insulating layer interposed between the first substrate and the second chip, a connection wiring structure disposed in the interlayer insulating layer, and a connection contact plug disposed in the interlayer insulating layer and connecting the connection wiring structure to the device isolation portion in the optical black region. The image sensor further includes a conductive pad disposed in the first chip or the second chip.