An image sensor may include unit pixel blocks with each having pixels for sensing incident light. Each unit pixel block may include a first sub pixel block including a first floating diffusion, a second sub pixel block including a second floating diffusion, and a common transistor block including a first drive transistor adjacent to the first floating diffusion and a second drive transistor adjacent to the second floating diffusion. The first and second floating diffusions may be electrically coupled in common to the first and second drive transistors.