An image sensor includes a stack structure including an active pixel region in which a plurality of pixels are defined, and a pad region arranged on at least one side of the active pixel region. The stack structure includes a first substrate including a photoelectric conversion region and a floating diffusion region in each pixel, a first semiconductor substrate, a first front structure on the first semiconductor substrate, and a pad opening penetrating the first semiconductor substrate in the pad region, a second substrate attached to the first substrate and including a pixel gate electrically connected to the floating diffusion region in each pixel, a third substrate attached to the second substrate and including a logic transistor for driving the plurality of pixels, and a pad having a top surface that is exposed through the pad opening.