An image sensor with improved performance, and a method of fabricating the same are provided. The image sensor includes a sensor array region and a pad region, which is disposed outside the sensor array region, the image sensor comprising a first substrate including a first surface, upon which light is incident, and a second surface, which is opposite to the first surface, a first isolation film in the first substrate at the sensor array region, the first isolation film defining a plurality of unit pixels, a second substrate including a third surface, which faces the second surface of the first substrate, and a fourth surface, which is opposite to the third surface, a wiring structure between the second and third surfaces, the wiring structure including an interlayer insulating film and a wiring in the interlayer insulating film, a pad trench in the pad region, the pad trench exposing the wiring through the first substrate, a bonding terminal in the pad trench, the bonding terminal being connected to the wiring, and a second isolation film in the first substrate at the pad region, the second isolation film being adjacent to the pad trench, wherein widths of each of the first and second isolation films decrease in a direction from the second surface to the first surface.