An image sensor for reducing channel variation and an image processing system including the same. The image sensor includes first to mth pixels (m≧2), each of which is connected to a corresponding column line from among first to mth column lines and is configured to output a respective pixel signal.’ The image sensor further includes first to mth bias circuits, each of which is connected to a corresponding column line from among the first to mth column lines and is configured to fix a voltage of the corresponding column line to a bias voltage when a column line-specific pixel is not selected to output the respective pixel signal. An analog-to-digital converter in the image sensor is configured to convert the pixel signals into digital signals.