An image sensor is provided. The image sensor includes a pixel configured to generate a reset signal and an image signal, a comparator configured to compare the reset signal with a reference signal and generate a first comparison signal, a counter configured to generate a first count value corresponding to the reset signal based on a clock signal and the first comparison signal, and a reference signal generator configured to generate the reference signal which changes between a first level corresponding to a maximum reset count value of the counter and a second level corresponding to a minimum reset count value of the counter during a reset signal period.