An image sensor may include a substrate having a first surface and a second surface on opposite sides, a first transistor having a first gate disposed on the first surface, a photoelectric conversion layer which generates photocharges from light incident in a first direction, a second transistor having a transistor structure disposed between the first surface and the photoelectric conversion layer and spaced from the photoelectric conversion layer, and includes a semiconductor layer composed of a metal oxide semiconductor material. The semiconductor layer may have a third surface facing the first direction and a fourth surface opposite the third surface, with a second gate disposed on the semiconductor layer. The semiconductor layer may be connected to the first gate. A light blocking layer may be disposed between the third surface and the photoelectric conversion layer, and spaced from the photoelectric conversion layer.