An X-ray detector including photodetecting pixels that reduce kTC switching noise. Each pixel includes a first transistor having a first electrode connected to a first power line, a second electrode connected to a first node, and a gate electrode receiving the reset signal; a second transistor having a first electrode connected to the first power line, a second electrode connected to a second node, and a gate electrode connected to the first node; a third transistor having a first electrode connected to the second node, a second electrode connected to a data line, and a gate electrode connected to a gate line; a fourth transistor having a first electrode connected to the first node, a second electrode connected to a photodetecting diode, and a gate electrode receiving the control signal; and the photodetecting diode having a first electrode connected to the fourth transistor, and a second electrode connected to ground.