An image sensor is provided. The image sensor according to an example embodiment includes a pixel array including a plurality of pixels, a ramp signal generator configured to generate a ramp signal that increases or decreases with a constant slope, a signal selector configured to output the ramp signal and one of a pixel signal and a test signal output from the pixel array, and a comparison circuit configured to receive the ramp signal and the test signal from the signal selector during a first period, output a first comparison result of the ramp signal and the test signal, receive the pixel signal and the ramp signal from the signal selector during a second period after the first period, and output a second comparison result of the pixel signal and the ramp signal.