An image sensor includes a semiconductor substrate having a first surface and a second surface opposite to the first surface, a transfer gate electrode provided on the first surface of the semiconductor substrate, readout circuit transistors spaced apart from the transfer gate electrode and provided on the first surface of the semiconductor substrate, and a photoelectric conversion layer provided in the semiconductor substrate at a side of the transfer gate electrode and including dopants of a first conductivity type. The photoelectric conversion layer includes a first region having a first thickness and a second region having a second thickness that is less than the first thickness. The second region overlaps with at least a portion of the readout circuit transistors in a direction perpendicular to the first surface of the semiconductor substrate.